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/admin_smtp.go

@@ -10,6 +10,7 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/mailer"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/secret"
 )
 
 type smtpConfigResp struct {
@@ -76,11 +77,15 @@ func (h *handlers) handleUpdateSMTPConfig(w http.ResponseWriter, r *http.Request
 		writeErrWithLog(w, h.logger, "admin smtp: get failed", apierror.Internal(err))
 		return
 	}
-	// Preserve existing password when the request sends "" (so the
-	// admin doesn't have to re-enter it on every save).
-	password := req.Password
-	if password == "" {
-		password = current.Password
+	// Preserve the stored (sealed) password when the request sends "", so
+	// the admin doesn't have to re-enter it on every save.
+	password := current.Password
+	if req.Password != "" {
+		password, err = secret.Seal(secret.SMTPPassword.Purpose(), "", req.Password)
+		if err != nil {
+			writeErrWithLog(w, h.logger, "admin smtp: seal password", apierror.Internal(err))
+			return
+		}
 	}
 	if err := q.UpdateSMTPConfig(r.Context(), dbq.UpdateSMTPConfigParams{
 		Enabled:     req.Enabled,