[NX-101 Issue] Refactor error handling to use consistent API error format

Replaced all inline error messages with the standardized `api_error` helper for consistent error response formatting. This improves clarity, maintainability, and ensures uniform error structures across the application. Updated logging for collector failures to include error class and switched to warning level for target unreachable scenarios.

backend/app/services/collector.py

@@ -195,6 +195,7 @@ async def collect_once() -> None:
         except (OSError, SQLAlchemyError, asyncpg.PostgresError, Exception) as exc:
             now = datetime.now(timezone.utc)
             current_error = str(exc)
+            error_class = exc.__class__.__name__
             state = _failure_state.get(target.id)
             if state is None:
                 _failure_state[target.id] = {
@@ -202,7 +203,13 @@ async def collect_once() -> None:
                     "last_log_at": now,
                     "error": current_error,
                 }
-                logger.exception("collector_error target=%s err=%s", target.id, exc)
+                logger.warning(
+                    "collector_target_unreachable target=%s error_class=%s err=%s consecutive_failures=%s",
+                    target.id,
+                    error_class,
+                    current_error,
+                    1,
+                )
                 continue
 
             count = int(state.get("count", 0)) + 1
@@ -220,9 +227,10 @@ async def collect_once() -> None:
             if should_log:
                 state["last_log_at"] = now
                 state["error"] = current_error
-                logger.error(
-                    "collector_error_throttled target=%s err=%s consecutive_failures=%s",
+                logger.warning(
+                    "collector_target_unreachable target=%s error_class=%s err=%s consecutive_failures=%s",
                     target.id,
+                    error_class,
                     current_error,
                     count,
                 )