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>

bin/animaltrack

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+exec python -m animaltrack.cli "$@"

bin/serve-dev

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+exec env CSRF_SECRET="dev-secret" DEV_MODE=true python -m animaltrack.cli serve "$@"