feat: migrate persistence from JSON to SQLModel (Phase 1)

- Integrated SQLModel with SQLite for robust data persistence
- Refactored UserManager and WatchlistManager to use SQL queries
- Migrated models to SQLModel with relationships and primary keys
- Updated test suite with in-memory database isolation
- Removed deprecated JSON storage files

requirements.txt

@@ -10,6 +10,7 @@ aiohttp==3.11.11
 beautifulsoup4==4.12.3
 lxml==5.3.0
 jieba==0.42.1
+sqlmodel==0.0.22
 
 # Testing dependencies
 pytest==8.3.4