feat: implement serve command with dev mode support

- CLI serve command now runs uvicorn with migrations
- Add dev_mode setting to bypass auth with default admin user
- Add bin/animaltrack wrapper for Nix environment
- Add bin/serve-dev for quick local development
- Update flake.nix shellHook for PYTHONPATH and bin PATH

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -80,6 +80,8 @@
         ];
 
         shellHook = ''
+          export PYTHONPATH="$PWD/src:$PYTHONPATH"
+          export PATH="$PWD/bin:$PATH"
           echo "AnimalTrack development environment ready!"
           echo "Run 'animaltrack serve' to start the app"
         '';