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/library/acoustid_settings.go

@@ -10,6 +10,7 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/secret"
 )
 
 // AcoustID lookup settings: a database row, changed without a restart, shared
@@ -97,8 +98,12 @@ func (s *AcoustIDSettingsService) Set(ctx context.Context, in AcoustIDSettingsUp
 	}
 	params := dbq.UpdateAcoustIDSettingsParams{Enabled: in.Enabled, MinScore: in.MinScore}
 	if in.APIKey != nil {
+		sealed, err := secret.Seal(secret.AcoustIDAPIKey.Purpose(), "", *in.APIKey)
+		if err != nil {
+			return AcoustIDSettings{}, fmt.Errorf("acoustid settings: seal key: %w", err)
+		}
 		params.SetApiKey = true
-		params.ApiKey = *in.APIKey
+		params.ApiKey = sealed
 	}
 	row, err := dbq.New(s.pool).UpdateAcoustIDSettings(ctx, params)
 	if err != nil {
@@ -126,7 +131,7 @@ func (s *AcoustIDSettingsService) Changed() <-chan struct{} {
 func acoustIDSettingsFromRow(row dbq.AcoustidSetting) AcoustIDSettings {
 	out := AcoustIDSettings{Enabled: row.Enabled, MinScore: row.MinScore, UpdatedAt: row.UpdatedAt.Time}
 	if row.ApiKey != nil {
-		out.APIKey = *row.ApiKey
+		out.APIKey = secret.Reveal(secret.AcoustIDAPIKey.Purpose(), "", *row.ApiKey)
 	}
 	return out
 }