feat: add Docker Compose setup with bind-mount appdata directory

- Dockerfile builds image with app source in /app_src
- docker-entrypoint.sh syncs code from image on each start, inits DB
- docker-compose.yml mounts ./appdata:/app for easy backup
- backend/init_db.py for safe DB init (no drop_all)
- backend/database.py supports DATABASE_URL env var
- .dockerignore excludes venv, __pycache__, *.db, .git

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

backend/database.py

@@ -1,10 +1,13 @@
 # backend/database.py
+import os
+
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker, scoped_session
 
-SQLALCHEMY_DATABASE_URL = "sqlite:///./app.db"  # или PostgreSQL URL
+SQLALCHEMY_DATABASE_URL = os.getenv("DATABASE_URL", "sqlite:///./app.db")
 
-engine = create_engine(SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False})
+connect_args = {"check_same_thread": False} if SQLALCHEMY_DATABASE_URL.startswith("sqlite") else {}
+engine = create_engine(SQLALCHEMY_DATABASE_URL, connect_args=connect_args)
 SessionLocal = scoped_session(sessionmaker(autocommit=False, autoflush=False, bind=engine))
 
 def get_db():