feat: postgresql adapter

.env.example

@@ -1,12 +1,20 @@
-# Database
+# Database backend — "sqlite" (default) or "postgres"
+DATABASE_BACKEND=sqlite
+
+# Option A: SQLite (default, zero external dependencies)
 DATABASE_URL=sqlite://movies.db
 
+# Option B: PostgreSQL
+# DATABASE_BACKEND=postgres
+# DATABASE_URL=postgres://user:password@localhost:5432/movies_diary
+
 # Authentication
 JWT_SECRET=change-me
 JWT_TTL_SECONDS=86400
 
-# OMDb metadata
+# OMDb/TMDB metadata
 OMDB_API_KEY=your-key
+TMDB_API_KEY=your-key
 
 # Poster storage — Option A (local) is active. To use S3, comment it out and uncomment Option B: