feat(tg-fitness-bot): add test suite with pre-commit hook

pytest + pytest-asyncio in flake.nix. 53 tests covering parser
(all formats, error cases) and db (CRUD, soft delete, update,
stats, pagination). Pre-commit hook runs tests when fitness bot
files are staged.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

flake.nix

@@ -17,6 +17,8 @@
           python-telegram-bot
           python-dotenv
           aiohttp
+          pytest
+          pytest-asyncio
         ]);
 
       in