refactor: Remove MongoDB support, simplify to SQLite-only architecture

- Remove pymongo dependency from pyproject.toml and update lock file
- Delete leggen/database/mongo.py implementation file
- Simplify DatabaseService to SQLite-only operations with default enabled
- Update CLI database utilities to remove MongoDB logic and imports
- Update documentation and configuration examples to reflect SQLite-only approach
- Update test fixtures and configuration tests for simplified database setup
- Change SQLite default from false to true for better user experience

This simplification reduces complexity, removes external database dependencies,
and focuses on the robust built-in SQLite solution. All 46 tests passing.

Benefits:
- Simpler architecture with single database solution
- Reduced dependencies (removed pymongo and dnspython)
- Cleaner configuration with less complexity
- Easier maintenance with fewer code paths

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

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

tests/conftest.py

@@ -29,8 +29,7 @@ def mock_config(temp_config_dir):
             "url": "https://bankaccountdata.gocardless.com/api/v2"
         },
         "database": {
-            "sqlite": True,
-            "mongodb": False
+            "sqlite": True
         },
         "scheduler": {
             "sync": {