feat: Implement core application structure, AI extraction, persistence, and Telegram bot modules with updated configuration and dependencies.

app/persistence/db.py

@@ -0,0 +1,46 @@
+"""
+Database connection and session management.
+"""
+from sqlalchemy import create_engine
+from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.orm import sessionmaker
+import logging
+
+from app.config import config
+
+logger = logging.getLogger(__name__)
+
+try:
+    # The 'check_same_thread' argument is specific to SQLite.
+    engine_args = {"check_same_thread": False} if config.DATABASE_URL.startswith("sqlite") else {}
+    
+    engine = create_engine(
+        config.DATABASE_URL,
+        connect_args=engine_args
+    )
+
+    SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+    Base = declarative_base()
+
+    logger.info("Database engine created successfully.")
+
+except Exception as e:
+    logger.critical(f"Failed to connect to the database: {e}")
+    # Exit or handle the critical error appropriately
+    engine = None
+    SessionLocal = None
+    Base = None
+
+def get_db():
+    """
+    Dependency for FastAPI routes to get a DB session.
+    """
+    if SessionLocal is None:
+        raise Exception("Database is not configured. Cannot create session.")
+    
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()