fix(submit): bound the untrusted upload lane — per-user caps + throttles (#75)

A logged-in user could file submissions without bound and stream a 1 GiB
context per submission. The only limits were the single-blob size cap and the
5 GiB uploads PVC (platform/workloads.go); nothing counted a user's rows or
bytes, so one account could fill the volume and every other user's upload
would start failing.

- Create: per-user pending_review cap (default 5) — the review queue cannot
  be parked full of one account's rows. Check-then-insert, documented soft.
- UploadContext: per-user stored-context budget (default 2 GiB) charged
  against the blob store's REAL sizes (new Blobs.Size on local/S3 stores), so
  the sum cannot drift from the volume; the write is capped at the remaining
  budget, so the excess is refused before it is persisted, and a re-upload is
  charged only for its new bytes.
- API: per-user create/upload throttles (30s/15s, cmd/felis-wired) on a
  dedicated cooldown keyspace, reserve→release so a failed attempt never
  burns the window and a burst collapses to one winner; ErrQuotaExceeded →
  403 submission_quota_exceeded (distinct from the 400 an oversize blob
  gets), 429 submission_cooldown for the throttles.
- Panel: zh/en copy for both codes; openapi documents 403/429 on the two
  user routes; pgint covers the pending-queue count.

Unit tests: submit package (cap, budget boundary/exact-fit/replacement,
oversize-vs-quota split) and api handlers (quota 403 both paths, throttle
429 + recovery + failure-release). go vet/go test/gofmt clean; panel
vitest 118 + typecheck green.

internal/submit/pgstore.go

@@ -33,6 +33,14 @@ func (s *PGStore) CreateSubmission(ctx context.Context, sub *Submission) error {
 	return err
 }
 
+func (s *PGStore) CountPendingSubmissionsBy(ctx context.Context, submittedBy string) (int, error) {
+	const q = `SELECT count(*) FROM image_submissions
+		WHERE submitted_by = $1 AND status = 'pending_review'`
+	var n int
+	err := s.db.QueryRowContext(ctx, q, submittedBy).Scan(&n)
+	return n, err
+}
+
 func (s *PGStore) GetSubmission(ctx context.Context, id string) (*Submission, error) {
 	const q = `SELECT ` + submissionColumns + ` FROM image_submissions WHERE id = $1`
 	return scanSubmission(s.db.QueryRowContext(ctx, q, id))