Fix health check endpoint to return 503 when fail2ban is offline

The health check endpoint now properly indicates service unavailability:
- Returns HTTP 200 when fail2ban is online
- Returns HTTP 503 when fail2ban is offline

This allows Docker and other orchestration tools to correctly detect when
fail2ban is unreachable and automatically restart the backend container,
preventing the situation where Docker treats the container as healthy
despite fail2ban being down.

Changes:
- Update GET /api/health to return 503 on fail2ban offline
- Return appropriate JSON response bodies for each state
- Update tests to verify both online (200) and offline (503) scenarios
- Update Dockerfile HEALTHCHECK documentation
- Add Health Checks section to Deployment.md documentation

All tests pass with 100% coverage on health.py.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docker/Dockerfile.backend

@@ -64,8 +64,10 @@ EXPOSE 8000
 USER bangui
 
 # Health-check using the built-in health endpoint
+# Returns exit 0 (success) for HTTP 200 (fail2ban online)
+# Returns exit 1 (failure) for HTTP 503 (fail2ban offline)
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
-    CMD python -c "import urllib.request; urllib.request.urlopen('http://localhost:8000/api/health')" || exit 1
+    CMD python -c "import urllib.request; urllib.request.urlopen('http://localhost:8000/api/health'); print('healthy')" || exit 1
 
 # ⚠️  IMPORTANT: Single-Worker Requirement
 # BanGUI must always run as a single worker process: