fix: resolve database and session initialization issues

- Switch from SQLAlchemy sessions to filesystem sessions to avoid race
  conditions with multiple gunicorn workers trying to create the sessions
  table simultaneously
- Update Alembic env.py to use a minimal Flask app without Flask-Session
  to prevent session table creation during migrations
- Add data directory creation to entrypoint.sh for clean container starts
- Configure test environment to use filesystem sessions with temp directory

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app.py

@@ -51,8 +51,10 @@ def create_app(config_name: str | None = None) -> Flask:
     bcrypt.init_app(app)
     csrf.init_app(app)
 
-    # Initialize session with SQLAlchemy backend
-    app.config["SESSION_SQLALCHEMY"] = db
+    # Initialize session with filesystem backend
+    # Ensure session directory exists
+    session_dir = Path(app.config.get("SESSION_FILE_DIR", data_dir / "sessions"))
+    session_dir.mkdir(parents=True, exist_ok=True)
     session.init_app(app)
 
     # Register blueprints