feat: add health endpoints, static serving, and base template (Step 7.2)

- Add /healthz endpoint with DB writable check
- Add /metrics endpoint with Prometheus-compatible format (enabled by default)
- Configure static file serving at /static/v1/... with immutable cache headers
- Create base page template with MonsterUI slate theme
- Create industrial farm aesthetic bottom navigation with custom SVG icons
- Add StaticCacheMiddleware for adding cache-control headers

Changes:
- src/animaltrack/web/routes/health.py: Health and metrics endpoints
- src/animaltrack/web/templates/: Base template, nav, and icons
- src/animaltrack/web/app.py: Integrate theme, routes, static serving
- src/animaltrack/config.py: metrics_enabled defaults to True

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

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

tests/test_config.py

@@ -81,7 +81,7 @@ class TestConfigDefaults:
             from animaltrack.config import Settings
 
             settings = Settings()
-            assert settings.metrics_enabled is False
+            assert settings.metrics_enabled is True
 
 
 class TestConfigEnvOverrides: