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/acquire/store_test.go

@@ -12,6 +12,7 @@ import (
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/dbtest"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/secret"
 )
 
 func newPool(t *testing.T) *pgxpool.Pool {
@@ -83,6 +84,15 @@ func TestStore_AddsConfiguresAndRemoves_Integration(t *testing.T) {
 	if err != nil || full.ApiKey != "key" {
 		t.Errorf("stored key = %q (err %v), want it trimmed", full.ApiKey, err)
 	}
+	// The column holds the key sealed, never the key.
+	var raw string
+	if err := s.pool.QueryRow(ctx,
+		"SELECT api_key FROM acquisition_services WHERE code = $1", first.Code).Scan(&raw); err != nil {
+		t.Fatalf("read api_key: %v", err)
+	}
+	if raw == "key" || !secret.IsSealed(raw) {
+		t.Errorf("api_key column = %q, want it sealed", raw)
+	}
 	rows, err := s.List(ctx)
 	if err != nil || len(rows) != 2 {
 		t.Fatalf("list: %d rows, err %v", len(rows), err)