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

Dockerfile

@@ -20,6 +20,7 @@ RUN uv sync --no-dev --frozen 2>/dev/null || uv sync --no-dev
 COPY wiregui/ wiregui/
 COPY alembic/ alembic/
 COPY alembic.ini ./
+COPY img/ img/
 
 FROM python:3.13-slim AS runner
 
@@ -34,6 +35,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 COPY --from=builder /usr/local/bin/uv /usr/local/bin/uv
 COPY --from=builder /app/.venv /app/.venv
 COPY --from=builder /app/wiregui /app/wiregui
+COPY --from=builder /app/img /app/img
 COPY --from=builder /app/alembic /app/alembic
 COPY --from=builder /app/alembic.ini /app/alembic.ini
 COPY --from=builder /app/pyproject.toml /app/pyproject.toml