feat: stored credentials are sealed at rest, so a copy of the database alone reveals none of them (#6767)

Minstrel keeps credentials it has to read back: indexer, Prowlarr and
download-client keys and passwords, the AcoustID key, cover-art and tag
provider keys, the SMTP password, and each user's ListenBrainz token and
Subsonic password. They were stored in the clear; they are now sealed with
AES-256-GCM under a per-column HKDF key derived from the stream secret, each
value bound to its row.

- internal/secret: Seal/Open/Reveal, and Columns, the one list of sealed
  columns (a backup will leave them out). Install takes the master key at
  startup; SealStored seals any value still in the clear, so an existing
  database is brought over in place on the first boot.
- Every write seals and every read opens: acquire (one read path, getService
  / usableServices), acoustid settings, SMTP, ListenBrainz, Subsonic auth and
  password generation, cover-art and tag provider settings.
- A value that does not open (the stream secret changed) is logged and reads
  as unset, to be entered again in Settings. docs/security.md says so.
- Tests: secret unit tests (wrong owner/purpose/key, malformed, no key), a
  SealStored integration test over all eight columns plus a schema check that
  each listed column exists, and the existing tests now assert sealed storage.
  dbtest installs a test key for integration tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/me_subsonic_password.go

@@ -8,6 +8,7 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/audit"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/secret"
 )
 
 // The Subsonic password is for clients that sign in with Subsonic's t/s
@@ -51,9 +52,15 @@ func (h *handlers) handleGenerateMySubsonicPassword(w http.ResponseWriter, r *ht
 		return
 	}
 	pw := base64.RawURLEncoding.EncodeToString(b)
+	sealed, err := secret.Seal(secret.SubsonicPassword.Purpose(), user.ID.String(), pw)
+	if err != nil {
+		h.logger.Error("generate subsonic password: seal failed", "err", err)
+		writeErr(w, apierror.Internal(err))
+		return
+	}
 	if err := dbq.New(h.pool).SetSubsonicPassword(r.Context(), dbq.SetSubsonicPasswordParams{
 		ID:               user.ID,
-		SubsonicPassword: &pw,
+		SubsonicPassword: &sealed,
 	}); err != nil {
 		h.logger.Error("generate subsonic password: update failed", "err", err)
 		writeErr(w, apierror.Internal(err))