fix(fc3i): single-line celery.signals import + INT32 bounds on target_id + dict.fromkeys + rewrite retry test as direct _finalize unit

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/celery_signals.py

@@ -22,9 +22,7 @@ import logging
 from datetime import UTC, datetime
 
 from celery.exceptions import SoftTimeLimitExceeded
-from celery.signals import (
-    task_failure, task_postrun, task_prerun, task_retry,
-)
+from celery.signals import task_failure, task_postrun, task_prerun, task_retry
 
 from .models import TaskRun
 from .tasks._sync_engine import sync_session_factory
@@ -44,6 +42,15 @@ _MAX_ERROR_MESSAGE_LEN = 2000
 _MAX_ARGS_SUMMARY_LEN = 255
 _MAX_WORKER_HOSTNAME_LEN = 128
 
+# PostgreSQL Integer is signed 32-bit. Tasks called with a first-arg
+# int outside this range (e.g. an absurdly large mock value, or a
+# string-of-digits coercible to int but bigger than 2^31-1) would crash
+# the INSERT with NumericValueOutOfRange. Bound the recorded value to
+# the column's range; values outside become None (target_id is
+# nullable, so this is safe).
+_INT32_MAX = 2_147_483_647
+_INT32_MIN = -2_147_483_648
+
 
 def _queue_for(task) -> str:
     """Reverse the task→queue routing from celery_app.task_routes.
@@ -68,14 +75,18 @@ def _queue_for(task) -> str:
 
 
 def _target_id_from_args(args) -> int | None:
-    """Best-effort: if the first positional arg parses as int, record
-    it as target_id (image_id, source_id, etc.). Never raises."""
+    """Best-effort: if the first positional arg parses as int AND fits
+    in the column's signed-32-bit range, record it as target_id
+    (image_id, source_id, etc.). Never raises."""
     if not args:
         return None
     try:
-        return int(args[0])
+        value = int(args[0])
     except (TypeError, ValueError):
         return None
+    if value < _INT32_MIN or value > _INT32_MAX:
+        return None
+    return value
 
 
 def _truncate(s, limit: int) -> str | None: