Implement per-file import for in-progress torrents

Files within a downloading torrent are now imported as soon as their
individual progress reaches 100%, without waiting for the full torrent
to complete. A new processed_files table (info_hash, file_name) tracks
which files have been handled to avoid redundant re-hashing each poll.

- db.py: add processed_files table, is_file_processed, record_processed_file
- qbit_client.py: replace get_completed_torrents with get_torrents(filter, category)
- stash_handler.py: extract process_torrent_files helper, daemon now polls
  both downloading and completed torrents each cycle

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

qbit_client.py

@@ -37,9 +37,11 @@ class QBitClient:
         resp.raise_for_status()
         return resp
 
-    def get_completed_torrents(self, category=None):
-        """Return a list of completed torrent dicts from qBittorrent."""
-        params = {"filter": "completed"}
+    def get_torrents(self, filter=None, category=None):
+        """Return a list of torrent dicts, optionally filtered by state and/or category."""
+        params = {}
+        if filter:
+            params["filter"] = filter
         if category:
             params["category"] = category
         resp = self._request("GET", "/api/v2/torrents/info", params=params)