fix: 时区统一北京时区 — Edge(UTC机器)三处时间处理修复

背景: Oracle Edge 机器为 UTC 时区，NAS 与视频均为北京时间，
网页要求统一北京时区。排查结论: NAS 端(北京时间)与 UI(直读
MariaDB)均正确，问题集中在 Edge 端三处。

1. queue_manager: 7 处 datetime('now','localtime') 在 UTC 机器上
   写入 UTC 时间(比北京慢8h)，全部改为 datetime('now','+8 hours')
2. preprocessor.compute_timestamps: event_start_time 缺失时 fallback
   datetime.now() 用了 Edge 本地时间(UTC)，改为北京时间
   datetime.now(timezone(+8h))；带时区的 ISO 输入统一转北京时间
3. nvidia_adapter._ts_to_seconds: 不支持生产格式
   'YYYY-MM-DD HH:MM:SS'(split后int抛ValueError全部返回-1)，
   导致集锦视频永远为空、视频模式永远降级逐帧——上一轮引入的
   bug，测试用 HH:MM:SS 格式未暴露。现支持两种格式；drawtext
   标签与提示词同步为完整时间戳说明

数据修正: event 17(task 298 手动测试缺 event_start_time)的
frame_timestamp 全为 UTC，按视频文件名真实时间(04:34:10)重算；
Edge 队列存量时间戳 +8h。

验证: 单测三例通过(生产格式解析/fallback北京时间/ISO带时区转换)；
task 41 新代码正确写入北京时间 17:36:52。

fam-edge/src/fam_edge/queue/queue_manager.py

@@ -47,8 +47,8 @@ def _init_db():
             error_message TEXT,
             failure_stage TEXT,
             retry_count INTEGER DEFAULT 0,
-            created_at TEXT DEFAULT (datetime('now', 'localtime')),
-            updated_at TEXT DEFAULT (datetime('now', 'localtime')),
+            created_at TEXT DEFAULT (datetime('now', '+8 hours')),
+            updated_at TEXT DEFAULT (datetime('now', '+8 hours')),
             delivered INTEGER DEFAULT 0,
             UNIQUE(nas_task_id)
         )
@@ -85,7 +85,7 @@ def enqueue(nas_task_id: int, video_filename: str, video_path: str,
                     "UPDATE task_queue SET status='PENDING', result_json=NULL, error_message=NULL, "
                     "failure_stage=NULL, retry_count=0, delivered=0, video_filename=?, video_path=?, "
                     "camera_name=?, event_start_time=?, known_members_context=?, "
-                    "updated_at=datetime('now','localtime') WHERE id=?",
+                    "updated_at=datetime('now','+8 hours') WHERE id=?",
                     (video_filename, video_path, camera_name, event_start_time,
                      known_members_context, row['id'])
                 )
@@ -105,7 +105,7 @@ def claim_next() -> Optional[Dict]:
         ).fetchone()
         if row:
             conn.execute(
-                "UPDATE task_queue SET status='PROCESSING', updated_at=datetime('now','localtime') WHERE id=?",
+                "UPDATE task_queue SET status='PROCESSING', updated_at=datetime('now','+8 hours') WHERE id=?",
                 (row['id'],)
             )
             conn.commit()
@@ -123,7 +123,7 @@ def mark_success(task_id: int, result_json: str):
     conn = _get_conn()
     try:
         conn.execute(
-            "UPDATE task_queue SET status='SUCCESS', result_json=?, updated_at=datetime('now','localtime') WHERE id=?",
+            "UPDATE task_queue SET status='SUCCESS', result_json=?, updated_at=datetime('now','+8 hours') WHERE id=?",
             (result_json, task_id)
         )
         conn.commit()
@@ -136,7 +136,7 @@ def mark_failed(task_id: int, error_message: str, failure_stage: str = ''):
     try:
         conn.execute(
             "UPDATE task_queue SET status='FAILED', error_message=?, failure_stage=?, "
-            "updated_at=datetime('now','localtime') WHERE id=?",
+            "updated_at=datetime('now','+8 hours') WHERE id=?",
             (error_message, failure_stage, task_id)
         )
         conn.commit()
@@ -165,7 +165,7 @@ def mark_delivered(task_ids: List[int]):
     try:
         placeholders = ','.join('?' * len(task_ids))
         conn.execute(
-            f"UPDATE task_queue SET delivered=1, updated_at=datetime('now','localtime') "
+            f"UPDATE task_queue SET delivered=1, updated_at=datetime('now','+8 hours') "
             f"WHERE id IN ({placeholders})", task_ids
         )
         conn.commit()