diff --git a/internal/api/api_test.go b/internal/api/api_test.go
index 468b2dd..a61c87f 100644
--- a/internal/api/api_test.go
+++ b/internal/api/api_test.go
@@ -1106,7 +1106,22 @@ func (f *fakeRepo) RedeemMigration(_ context.Context, targetUserID, codeHash str
 
 // ---- quota admin fakes ----
 
+// liveUserExists mirrors PGRepo.requireLiveUser: the admin quota/link fakes
+// only act on a live seeded row, so a unit test can drive the unknown-user 404
+// the real FK would otherwise turn into a 500.
+func (f *fakeRepo) liveUserExists(id string) bool {
+	for _, su := range f.seededUsers {
+		if su.view.ID == id && su.detail.DeletedAt == nil {
+			return true
+		}
+	}
+	return false
+}
+
 func (f *fakeRepo) GetQuotas(_ context.Context, userID string) (*QuotaView, error) {
+	if !f.liveUserExists(userID) {
+		return nil, ErrNotFound
+	}
 	v := &QuotaView{UserID: userID}
 	if f.fakeQuotas == nil {
 		return v, nil
@@ -1121,6 +1136,9 @@ func (f *fakeRepo) GetQuotas(_ context.Context, userID string) (*QuotaView, erro
 }
 
 func (f *fakeRepo) SetQuotas(_ context.Context, userID string, qi QuotaInput, _ string) (*QuotaView, error) {
+	if !f.liveUserExists(userID) {
+		return nil, ErrNotFound
+	}
 	if f.fakeQuotas == nil {
 		f.fakeQuotas = map[string]*QuotaView{}
 	}
@@ -1173,6 +1191,9 @@ func (f *fakeRepo) UnlinkAccount(_ context.Context, userID, mcUUID string) error
 }
 
 func (f *fakeRepo) LinkAccount(_ context.Context, userID, mcUUID, authSource string) error {
+	if !f.liveUserExists(userID) {
+		return ErrNotFound
+	}
 	if existing, ok := f.links[mcUUID]; ok && existing != userID {
 		return ErrConflict
 	}
diff --git a/internal/api/handlers_users.go b/internal/api/handlers_users.go
index c01cf45..80f62c5 100644
--- a/internal/api/handlers_users.go
+++ b/internal/api/handlers_users.go
@@ -281,6 +281,10 @@ func (a *API) handleGetQuotas(w http.ResponseWriter, r *http.Request) {
 	}
 	v, err := a.Repo.GetQuotas(r.Context(), id)
 	if err != nil {
+		if errors.Is(err, ErrNotFound) {
+			writeError(w, r, newError(http.StatusNotFound, "not_found", "user not found"))
+			return
+		}
 		writeError(w, r, err)
 		return
 	}
@@ -477,6 +481,10 @@ func (a *API) handleLinkAccount(w http.ResponseWriter, r *http.Request) {
 				"this UUID is already linked to a different user"))
 			return
 		}
+		if errors.Is(err, ErrNotFound) {
+			writeError(w, r, newError(http.StatusNotFound, "not_found", "user not found"))
+			return
+		}
 		writeError(w, r, err)
 		return
 	}
diff --git a/internal/api/handlers_users_test.go b/internal/api/handlers_users_test.go
index 9fc2745..a18ec26 100644
--- a/internal/api/handlers_users_test.go
+++ b/internal/api/handlers_users_test.go
@@ -51,3 +51,49 @@ func TestOwnerAccountProtectedFromPanelMutations(t *testing.T) {
 		}
 	})
 }
+
+// The user-scoped admin sub-resources (quotas, account links) must answer 404
+// for an unknown user id. Before the requireLiveUser guard the quota upsert and
+// the link insert reached the users(id) foreign key and surfaced as an opaque
+// 500 (found live against the drill cluster, audit #30), and the quotas read
+// answered a zero-value "unlimited" view as if the id existed.
+func TestAdminSubresourcesRequireLiveUser(t *testing.T) {
+	owner := &Principal{UserID: "usr-root", Role: "owner", ViaAdminAccess: true}
+	repo := newFakeRepo()
+	repo.seedUser(UserView{ID: "usr-root", Username: "root", Role: "owner"})
+	repo.seedUser(UserView{ID: "u2", Username: "alice", Role: "user"})
+	api := newTestAPI(repo, newFakeCluster())
+	api.External = staticExternal{p: owner}
+	eh := api.ExternalHandler()
+
+	t.Run("quotas read of an unknown user", func(t *testing.T) {
+		w := do(eh, "GET", "/api/v1/users/usr-nope/quotas", "", nil)
+		if w.Code != http.StatusNotFound {
+			t.Fatalf("code = %d body %s, want 404", w.Code, w.Body.String())
+		}
+	})
+	t.Run("quotas write of an unknown user", func(t *testing.T) {
+		w := do(eh, "PUT", "/api/v1/users/usr-nope/quotas", `{"max_servers":1}`, jsonHeader)
+		if w.Code != http.StatusNotFound {
+			t.Fatalf("code = %d body %s, want 404", w.Code, w.Body.String())
+		}
+	})
+	t.Run("account link of an unknown user", func(t *testing.T) {
+		w := do(eh, "POST", "/api/v1/users/usr-nope/links",
+			`{"mc_uuid":"22222222-3333-4444-5555-666666666666"}`, jsonHeader)
+		if w.Code != http.StatusNotFound {
+			t.Fatalf("code = %d body %s, want 404", w.Code, w.Body.String())
+		}
+	})
+	t.Run("control: a live user still accepts both writes", func(t *testing.T) {
+		w := do(eh, "PUT", "/api/v1/users/u2/quotas", `{"max_servers":2}`, jsonHeader)
+		if w.Code != http.StatusOK {
+			t.Fatalf("set quotas: code = %d body %s, want 200", w.Code, w.Body.String())
+		}
+		w = do(eh, "POST", "/api/v1/users/u2/links",
+			`{"mc_uuid":"22222222-3333-4444-5555-666666666677"}`, jsonHeader)
+		if w.Code != http.StatusOK {
+			t.Fatalf("link: code = %d body %s, want 200", w.Code, w.Body.String())
+		}
+	})
+}
diff --git a/internal/api/pgrepo.go b/internal/api/pgrepo.go
index de68490..a1a7728 100644
--- a/internal/api/pgrepo.go
+++ b/internal/api/pgrepo.go
@@ -1632,8 +1632,13 @@ func (p *PGRepo) SetUserDisabled(ctx context.Context, userID string, disabled bo
 // ---- quota admin ----
 
 // GetQuotas returns the quotas row for a user, or a zero-value view when no
-// row exists (meaning unlimited).
+// row exists (meaning unlimited). An unknown or soft-deleted user id is
+// ErrNotFound, never a zero-value "unlimited" answer — the admin sub-resource
+// routes all 404 on a user that has no live row.
 func (p *PGRepo) GetQuotas(ctx context.Context, userID string) (*QuotaView, error) {
+	if err := p.requireLiveUser(ctx, userID); err != nil {
+		return nil, err
+	}
 	const q = `SELECT user_id, max_servers, max_cpu_milli, max_memory_mb, max_storage_gb
 		FROM quotas WHERE user_id = $1`
 	v := QuotaView{UserID: userID}
@@ -1647,9 +1652,29 @@ func (p *PGRepo) GetQuotas(ctx context.Context, userID string) (*QuotaView, erro
 	return &v, nil
 }
 
+// requireLiveUser gates the user-scoped admin sub-resources (quotas, account
+// links) on a live users row. Without it a write would hit the user_id foreign
+// key and surface as an opaque 500, and the read would answer as if a
+// never-existed id did; every admin route answers ErrNotFound → 404 instead.
+func (p *PGRepo) requireLiveUser(ctx context.Context, userID string) error {
+	var ok bool
+	if err := p.db.QueryRowContext(ctx,
+		`SELECT EXISTS(SELECT 1 FROM users WHERE id = $1 AND deleted_at IS NULL)`,
+		userID).Scan(&ok); err != nil {
+		return err
+	}
+	if !ok {
+		return ErrNotFound
+	}
+	return nil
+}
+
 // SetQuotas upserts a quotas row. Nil fields are left unchanged; a non-nil
 // zero-value field clears the cap.
 func (p *PGRepo) SetQuotas(ctx context.Context, userID string, qi QuotaInput, setBy string) (*QuotaView, error) {
+	if err := p.requireLiveUser(ctx, userID); err != nil {
+		return nil, err
+	}
 	type col struct {
 		name  string
 		value *int
@@ -1754,6 +1779,9 @@ func (p *PGRepo) UnlinkAccount(ctx context.Context, userID, mcUUID string) error
 // NOTHING on the UNIQUE(mc_uuid) constraint, plus an idempotency check via
 // EXISTS).
 func (p *PGRepo) LinkAccount(ctx context.Context, userID, mcUUID, authSource string) error {
+	if err := p.requireLiveUser(ctx, userID); err != nil {
+		return err
+	}
 	// Check idempotency first: already linked to this user → success.
 	var exists bool
 	if err := p.db.QueryRowContext(ctx,
diff --git a/internal/pgint/pgint_test.go b/internal/pgint/pgint_test.go
index 1d580df..be49d75 100644
--- a/internal/pgint/pgint_test.go
+++ b/internal/pgint/pgint_test.go
@@ -439,6 +439,48 @@ func TestUserAdminEmailEditClearsVerification(t *testing.T) {
 	assertEmailProven(t, u.ID, next, false)
 }
 
+// The user-scoped admin sub-resources must refuse an id that has no live users
+// row with ErrNotFound (→ the API's 404). The quota upsert and the account-link
+// insert touch user_id foreign keys, so before the requireLiveUser guard the
+// live drill returned a 500 on both (audit #30); the quotas read answered a
+// zero-value "unlimited" view for an id that never existed.
+func TestAdminSubresourcesRequireLiveUser(t *testing.T) {
+	ctx := context.Background()
+	ghost := "usr-ghost-" + suffix(t)
+
+	if _, err := repo.GetQuotas(ctx, ghost); !errors.Is(err, api.ErrNotFound) {
+		t.Fatalf("GetQuotas(ghost) = %v, want ErrNotFound", err)
+	}
+	three := 3
+	if _, err := repo.SetQuotas(ctx, ghost, api.QuotaInput{MaxServers: &three}, "pgint"); !errors.Is(err, api.ErrNotFound) {
+		t.Fatalf("SetQuotas(ghost) = %v, want ErrNotFound", err)
+	}
+	if err := repo.LinkAccount(ctx, ghost, testUUID(t), "mojang"); !errors.Is(err, api.ErrNotFound) {
+		t.Fatalf("LinkAccount(ghost) = %v, want ErrNotFound", err)
+	}
+
+	// Control: the same calls land for a live user.
+	u := newUser(t, "user", "subres")
+	if _, err := repo.SetQuotas(ctx, u.ID, api.QuotaInput{MaxServers: &three}, "pgint"); err != nil {
+		t.Fatalf("SetQuotas(live): %v", err)
+	}
+	got, err := repo.GetQuotas(ctx, u.ID)
+	if err != nil || got.MaxServers == nil || *got.MaxServers != 3 {
+		t.Fatalf("GetQuotas(live) = %+v, %v; want max_servers=3", got, err)
+	}
+	if err := repo.LinkAccount(ctx, u.ID, testUUID(t), "mojang"); err != nil {
+		t.Fatalf("LinkAccount(live): %v", err)
+	}
+
+	// A soft-deleted user is no longer a live target either.
+	if err := repo.DeleteUser(ctx, u.ID, "pgint"); err != nil {
+		t.Fatalf("DeleteUser: %v", err)
+	}
+	if _, err := repo.SetQuotas(ctx, u.ID, api.QuotaInput{MaxServers: &three}, "pgint"); !errors.Is(err, api.ErrNotFound) {
+		t.Fatalf("SetQuotas(deleted) = %v, want ErrNotFound", err)
+	}
+}
+
 // The owner tier the panel gates on must actually be WRITTEN: until this
 // contract had a test, every provisioning path wrote 'admin', so the whole
 // owner surface (user administration) was unreachable in a fresh install.