Add logging for failure recovery and throttled error reporting.

Introduced `_failure_state` to track consecutive failures per target and `_failure_log_interval_seconds` to control logging frequency. Added logs for recovery when a target recovers and throttled error logs to reduce noise for recurring errors.

backend/app/services/collector.py

@@ -13,6 +13,8 @@ import asyncpg
 
 logger = logging.getLogger(__name__)
 settings = get_settings()
+_failure_state: dict[int, dict[str, object]] = {}
+_failure_log_interval_seconds = 300
 
 
 def build_target_dsn(target: Target) -> str:
@@ -182,8 +184,48 @@ async def collect_once() -> None:
     for target in targets:
         try:
             await collect_target(target)
+            prev = _failure_state.pop(target.id, None)
+            if prev:
+                logger.info(
+                    "collector_target_recovered target=%s after_failures=%s last_error=%s",
+                    target.id,
+                    prev.get("count", 0),
+                    prev.get("error"),
+                )
         except (OSError, SQLAlchemyError, asyncpg.PostgresError, Exception) as exc:
-            logger.exception("collector_error target=%s err=%s", target.id, exc)
+            now = datetime.now(timezone.utc)
+            current_error = str(exc)
+            state = _failure_state.get(target.id)
+            if state is None:
+                _failure_state[target.id] = {
+                    "count": 1,
+                    "last_log_at": now,
+                    "error": current_error,
+                }
+                logger.exception("collector_error target=%s err=%s", target.id, exc)
+                continue
+
+            count = int(state.get("count", 0)) + 1
+            last_log_at = state.get("last_log_at")
+            last_logged_error = str(state.get("error", ""))
+            should_log = False
+            if current_error != last_logged_error:
+                should_log = True
+            elif isinstance(last_log_at, datetime):
+                should_log = (now - last_log_at).total_seconds() >= _failure_log_interval_seconds
+            else:
+                should_log = True
+
+            state["count"] = count
+            if should_log:
+                state["last_log_at"] = now
+                state["error"] = current_error
+                logger.error(
+                    "collector_error_throttled target=%s err=%s consecutive_failures=%s",
+                    target.id,
+                    current_error,
+                    count,
+                )
 
 
 async def collector_loop(stop_event: asyncio.Event) -> None: