feat: WireGuard metrics collector + integration test stack

Metrics collector (wiregui/collector.py):
- Standalone process spawned by web app when WG_METRICS_ENABLED=true
- Polls wg show dump every WG_METRICS_POLL_INTERVAL seconds (default 5)
- Updates device stats in PostgreSQL
- Pushes Prometheus-format metrics to VictoriaMetrics (if configured)
- Graceful shutdown on SIGTERM

Integration test stack (compose.yml):
- Unified compose file for dev, test, and integration modes
- VictoriaMetrics single-node TSDB for metrics storage
- 3 mock WireGuard client containers generating ping traffic
- Automated setup script seeds server keypair, admin user, client devices
- make test-stack-up: one command to start everything
- make test-stack-verify: validates metrics flowing end-to-end

Infrastructure:
- Makefile with targets for dev, test, integration, and production
- Integration tests verify VictoriaMetrics has data for all 3 clients
- Fix Dockerfile to include img/ directory
- Separate TESTS.md for test tracking, clean TODO.md for features only

.gitignore

@@ -6,3 +6,4 @@ __pycache__/
 logs/
 .idea/
 .coverage
+docker/mock-clients/configs/