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>

README.md

@@ -17,7 +17,6 @@ Having your bank data accessible through both CLI and REST API gives you the pow
 
   ### 📦 Storage
   - [SQLite](https://www.sqlite.org): for storing transactions, simple and easy to use
-  - [MongoDB](https://www.mongodb.com/docs/): alternative store for transactions, good balance between performance and query capabilities
 
   ### 📊 Visualization
   - [NocoDB](https://github.com/nocodb/nocodb): for visualizing and querying transactions, a simple and easy to use interface for SQLite
@@ -32,7 +31,7 @@ Having your bank data accessible through both CLI and REST API gives you the pow
 - Support for both booked and pending transactions
 
 ### 🔄 Data Management
-- Sync all transactions with SQLite and/or MongoDB databases
+- Sync all transactions with SQLite database
 - Background sync scheduling with configurable cron expressions
 - Automatic transaction deduplication and status tracking
 - Real-time sync status monitoring
@@ -105,11 +104,6 @@ url = "https://bankaccountdata.gocardless.com/api/v2"
 
 [database]
 sqlite = true
-mongodb = false
-
-# Optional: MongoDB configuration
-[database.mongodb]
-uri = "mongodb://localhost:27017"
 
 # Optional: Background sync scheduling
 [scheduler.sync]