feat: add configuration management

- Add Settings class using pydantic-settings
- Load config from environment variables with GM_ prefix
- Support encrypt_key and api_token (required, no defaults for security)
- Provide defaults for data_dir, host, port
- Add computed properties for db_path, ssh_keys_dir, repos_dir
- Add tests for config defaults and environment variable overrides
- Add Base class to app.models to unblock conftest.py imports

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

backend/app/models/__init__.py

@@ -0,0 +1,11 @@
+"""ORM Models.
+
+NOTE: This module is a placeholder until Task 2.1.
+The Base class is needed by conftest.py for database fixtures.
+"""
+
+from sqlalchemy.orm import DeclarativeBase
+
+class Base(DeclarativeBase):
+    """Base class for all ORM models."""
+    pass