refactor(api): consolidate health check endpoints to /health

Removes the redundant `/healthz` and `/api/v1/health` endpoints, leaving `/health` as the sole health check endpoint.

- Update router to return 404 Not Found for the removed endpoints
- Update admin log filtering to only ignore `/health`
- Remove health URL from API documentation data
- Update tests to verify `/health` returns 200 and others return 404
- Update README documentation to reflect the change

backend/libs/handlers/admin.go

@@ -1770,7 +1770,7 @@ func isHealthCheckLogEntry(raw map[string]any) bool {
 	if idx := strings.IndexByte(path, '?'); idx >= 0 {
 		path = path[:idx]
 	}
-	return path == "/health" || path == "/healthz" || path == "/api/v1/health"
+	return path == "/health"
 }
 
 func logEntryFromMap(raw map[string]any) adminLogEntry {