feat: add Android app and end-to-end encrypted library sync

Introduce the Kotlin/Compose Android companion app and extend the TMDB
proxy into a combo server that synchronizes the whole library as an
encrypted snapshot, with all three components (Go server, Android,
desktop) sharing one zero-knowledge crypto envelope and JSON schema.

- server: add PostgreSQL-backed /sync/library (GET/PUT) with optimistic
  concurrency (revision + 409 on conflict); sync stays disabled unless
  DATABASE_URL is set. Add docker-compose with Postgres.
- desktop: add libsodium CryptoEnvelope, LibrarySerializer and SyncClient;
  storage-mode and passphrase settings; a "Sync now" toolbar action with
  conflict resolution.
- android: Room-backed library with local/cloud storage modes, encrypted
  sync client, and settings UI. The proxy server (URL + token) can be
  configured as a TMDB proxy even in local-only mode.

Crypto is identical across platforms: Argon2id (libsodium crypto_pwhash,
INTERACTIVE) + XChaCha20-Poly1305 in a versioned "UMTS" envelope, so a
snapshot encrypted on one client decrypts on the other.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/config.go

@@ -22,6 +22,7 @@ type Config struct {
 	SessionKey   []byte        // HMAC secret for signing access tokens / state cookies
 	AllowGroups  []string      // if set, the user must be in one of these Authentik groups
 	TokenTTL     time.Duration // lifetime of issued desktop-app tokens
+	DatabaseURL  string        // PostgreSQL DSN for library sync; empty disables sync
 }
 
 func getenv(key, def string) string {
@@ -41,6 +42,7 @@ func loadConfig() (*Config, error) {
 		ClientID:     getenv("OIDC_CLIENT_ID", ""),
 		ClientSecret: getenv("OIDC_CLIENT_SECRET", ""),
 		RedirectURL:  getenv("OIDC_REDIRECT_URL", ""),
+		DatabaseURL:  getenv("DATABASE_URL", ""),
 	}
 
 	if c.RedirectURL == "" {