refactor(download): 移除下载后自动删除逻辑，保留手动删除接口

1. download.py:
   - 完全移除 _mark_task_downloaded() 函数（原功能：下载后设置 delete_after_at
     触发 janitor 到期自动清理文件）
   - 改为仅记录 downloaded_at 时间戳，不再调度自动删除
   - 手动 DELETE /task/<id> 接口不受影响（tasks.py:151-211）

2. gasflux.ini:
   - successful_task_cleanup_age: 60 → 315360000（10年）
     彻底禁止 janitor 的下载后自动清理行为

src/gasflux/blueprints/download.py

@@ -3,6 +3,7 @@ Download Blueprint
 Handles file download endpoints.
 """
 
+from datetime import datetime
 from pathlib import Path
 from flask import Blueprint, send_file, current_app
 
@@ -10,40 +11,6 @@ from ..shared import _format_response, log_performance, logger
 from ..auth import require_api_key
 
 
-def _mark_task_downloaded(task_id):
-    """Mark task as downloaded and schedule deletion in database."""
-    import sqlite3
-    from pathlib import Path
-
-    # Use independent database connection (not from flask.g which may be closed)
-    from ..db import get_db_path as get_config_db_path
-    db_path = get_config_db_path(current_app)
-
-    # Get cleanup age for successful tasks from config (in seconds)
-    successful_task_cleanup_age = current_app.config.get('SUCCESSFUL_TASK_CLEANUP_AGE', 3600)
-
-    try:
-        conn = sqlite3.connect(str(db_path), check_same_thread=False)
-        conn.execute("PRAGMA foreign_keys=ON")
-        conn.execute("PRAGMA busy_timeout=3000")
-
-        # Update downloaded timestamp and set deletion time based on config
-        conn.execute("""
-            UPDATE tasks
-            SET downloaded_at = datetime('now', '+8 hours'),
-                delete_after_at = datetime('now', '+8 hours', '+' || ? || ' seconds')
-            WHERE task_id = ?
-        """, (successful_task_cleanup_age, task_id))
-
-        conn.commit()
-        logger.info(f"Task {task_id} marked as downloaded, scheduled for deletion in {successful_task_cleanup_age} seconds")
-
-    except Exception as e:
-        logger.error(f"Failed to mark task {task_id} as downloaded: {str(e)}", exc_info=True)
-    finally:
-        if 'conn' in locals():
-            conn.close()
-
 # Create blueprint
 download_bp = Blueprint('download', __name__, url_prefix='/download')
 
@@ -124,12 +91,19 @@ def download_file(filename):
         file_size = file_path.stat().st_size
         logger.info(f"Serving file: {filename} ({file_size} bytes)")
 
-        # Mark download immediately before sending file
+        # 记录下载时间（仅时间戳，不设置自动删除）
         if task_id:
             try:
-                _mark_task_downloaded(task_id)
+                from ..db import get_db
+                db = get_db()
+                db.execute(
+                    "UPDATE tasks SET downloaded_at = datetime('now', '+8 hours') WHERE task_id = ?",
+                    (task_id,)
+                )
+                db.commit()
+                logger.info(f"Task {task_id} downloaded at {datetime.now()}")
             except Exception as e:
-                logger.error(f"Failed to mark download for task {task_id}: {str(e)}")
+                logger.error(f"Failed to record download for task {task_id}: {str(e)}")
 
         response = send_file(file_path)
         return response