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/.env.example

@@ -31,3 +31,13 @@ SESSION_SECRET=change-me-to-a-long-random-string
 
 # How long an issued desktop-app token stays valid (in days).
 TOKEN_TTL_DAYS=90
+
+# --- Library-Sync (optional) -------------------------------------------
+# PostgreSQL-DSN für den verschlüsselten Bibliotheks-Sync. Leer lassen, um
+# den Sync zu deaktivieren (dann läuft der Server als reiner TMDB-Proxy).
+# Der Server speichert nur Chiffretext (Ende-zu-Ende, Zero-Knowledge).
+# Bei Nutzung von docker-compose.yml wird dies automatisch gesetzt.
+# DATABASE_URL=postgres://umt:change-me@db:5432/umt?sslmode=disable
+
+# Passwort des Postgres-Containers (nur für docker-compose.yml relevant).
+POSTGRES_PASSWORD=change-me