feat(api): add passkey enrollment persistence layer

Phase 6 WebAuthn bind, enrollment-only slice (spec section 14). Adds the data
layer an already-authenticated principal needs to bind and manage passkeys:

- migration 0007: webauthn_credentials (one bound passkey per row, public
  attestation material only) and webauthn_challenges (server-stashed ceremony
  state between begin and finish, single-use via consumed_at). Both rows are
  bound to a known user_id; there is no usernameless login lookup, since the
  assertion/login path is a deferred slice.
- PasskeyCredential type and five Repo methods (create/consume challenge,
  create/list/delete credential) with the PG semantics the handlers rely on:
  supersede-prior-live on begin, expiry-before-consume single-use on finish,
  credential_id UNIQUE -> ErrConflict, owner-scoped delete -> ErrNotFound.
- ErrPasskeyChallengeInvalid sentinel for a missing/expired/consumed ceremony.

internal/store/migrations/0007_webauthn_credentials.sql

@@ -0,0 +1,58 @@
+-- Phase 6 passkey/WebAuthn bind (spec §14 WebAuthn, §B2 onboarding "link + email-OTP
+-- + passkey"). This is the ENROLLMENT data layer only: an already-authenticated
+-- principal binds a passkey to their account (the credential-creation ceremony),
+-- and manages the credentials they have bound. Email-OTP remains the fallback
+-- factor (migration 0004), so a player with no passkey is never locked out.
+--
+-- Scope boundary (deliberate): this slice covers ENROLLMENT only. Assertion
+-- verification for LOGIN — proving a passkey to mint/elevate a session from an
+-- unauthenticated state — is out of scope here and deferred. The spec keeps the two
+-- passkey surfaces distinct (§14: admin.* rides "Tunnel+Access，WebAuthn/posture",
+-- panel.* is "app 登录"; §B2 lists passkey among the player onboarding factors), so
+-- where the panel.* passkey relying-party boundary ultimately lands (felis-api vs.
+-- the Access edge) is a later decision, not settled by this migration. Accordingly
+-- this migration models only the authenticated enrollment ceremony: every challenge
+-- is bound to a known user_id, and there is no usernameless (pre-session) login
+-- lookup column. Adding a login path later would also need the felis_session
+-- honoring model in session.go extended.
+
+-- webauthn_credentials stores one bound passkey per row. The public key and the
+-- signature counter are attestation outputs captured at registration; only public,
+-- non-secret material is held (a WebAuthn public key is meant to be public, unlike
+-- the RCON password or a session token). credential_id is the authenticator's
+-- globally-unique handle, base64url-encoded; UNIQUE guards the (astronomically
+-- unlikely) cross-account collision and is the key a future login path would match.
+CREATE TABLE webauthn_credentials (
+  id            text PRIMARY KEY,                      -- opaque row id (crypto-random hex)
+  user_id       text NOT NULL REFERENCES users(id),
+  credential_id text NOT NULL UNIQUE,                  -- base64url(raw credential id)
+  public_key    text NOT NULL,                         -- base64(COSE public key bytes)
+  sign_count    bigint NOT NULL DEFAULT 0,             -- uint32 widened (overflows int4)
+  aaguid        text,                                  -- authenticator model id, for display only
+  name          text,                                  -- caller-supplied nickname ("My phone")
+  created_at    timestamptz NOT NULL DEFAULT now(),
+  last_used_at  timestamptz                            -- NULL until an assertion is verified (deferred login path)
+);
+
+-- The credential-management views list a user's bound passkeys, so index that lookup.
+CREATE INDEX webauthn_credentials_user_id_idx ON webauthn_credentials (user_id);
+
+-- webauthn_challenges holds the server-side ceremony state between begin and finish.
+-- The full go-webauthn SessionData blob (challenge, allowed credentials, expiry) is
+-- stashed here and reloaded at finish, so the client never echoes — and so cannot
+-- forge — the challenge it must answer (the same principle as email_otps.code_hash).
+-- Every row is bound to a known user_id (NOT NULL): enrollment always rides on an
+-- authenticated principal, so there is no usernameless consume-by-hash path. consumed_at
+-- enforces single-use; a fresh begin supersedes the prior live row for (user, purpose).
+CREATE TABLE webauthn_challenges (
+  id           text PRIMARY KEY,                       -- opaque row id (crypto-random hex)
+  user_id      text NOT NULL REFERENCES users(id),
+  purpose      text NOT NULL,                          -- 'passkey_register'
+  session_data bytea NOT NULL,                         -- opaque go-webauthn SessionData
+  expires_at   timestamptz NOT NULL,                   -- short TTL set by the API clock
+  consumed_at  timestamptz,                            -- non-NULL once redeemed (single-use)
+  created_at   timestamptz NOT NULL DEFAULT now()
+);
+
+-- The finish path consumes the newest live row for a (user, purpose), so index it.
+CREATE INDEX webauthn_challenges_user_purpose_idx ON webauthn_challenges (user_id, purpose);