fix: dispatcher crash on invalid failure_stage + per-task error isolation

Two fixes:
1. db_layer.update_task_status: map invalid failure_stage values (e.g.
   'process' from Edge) to 'callback' before DB write, preventing
   MariaDB ENUM DataError (1265 "Data truncated")
2. dispatcher._poll_once: wrap each task dispatch in try/except so one
   task's failure doesn't skip remaining tasks in the batch

Root cause: Edge returns failure_stage='process' but DB ENUM only allows
  download/extract/vlm_visual/vlm_fusion/callback. The DataError crashed
  _poll_once(), causing all subsequent PENDING tasks to be skipped.

fam-core/src/fam_core/db_layer.py

@@ -92,6 +92,9 @@ def get_tasks_by_status(status: str, limit=10) -> List[Dict]:
 def update_task_status(task_id: int, status: str, error_message: str = None,
                        failure_stage: str = None):
     """更新任务状态"""
+    valid_stages = {'download', 'extract', 'vlm_visual', 'vlm_fusion', 'callback'}
+    if failure_stage and failure_stage not in valid_stages:
+        failure_stage = 'callback'
     conn = get_conn()
     try:
         cursor = conn.cursor()

fam-core/src/fam_core/dispatcher/dispatcher.py

@@ -165,7 +165,10 @@ class Dispatcher:
         tasks = db_layer.get_pending_tasks(limit=10)
         for task in tasks:
             if self._should_retry(task):
+                try:
                     self._dispatch_one(task)
+                except Exception as e:
+                    logger.error(f"[task_id={task['task_id']}] dispatch 异常: {e}", exc_info=True)
 
     def _run(self):
         """线程主循环"""