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>

pyproject.toml

@@ -11,7 +11,6 @@ keywords = [
     "cli",
     "psd2",
     "gocardless",
-    "mongodb",
     "bank",
     "transactions",
     "finance",
@@ -29,7 +28,6 @@ dependencies = [
     "requests>=2.31.0,<3",
     "loguru>=0.7.2,<0.8",
     "tabulate>=0.9.0,<0.10",
-    "pymongo>=4.6.1,<5",
     "discord-webhook>=1.3.1,<2",
     "fastapi>=0.104.0,<1",
     "uvicorn[standard]>=0.24.0,<1",