Smarter keeper selection: folder-name + mtime signals

Adds a path-penalty score that downranks files in folders named Trashed,
Dups, Backup, Copy, Old, Archive, plus a penalty for repeated path segments
(e.g. Desktop/Desktop/Files) and very deep paths. Also captures and uses
file mtime as a tiebreaker — older files are usually the originals.

Applied to all four detection passes (sha256, phash, exif, filesize+dim)
and to auto-resolve-exact.

New file_mtime column with idempotent migration.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/main.py

@@ -490,7 +490,8 @@ def auto_resolve_exact():
 
     for gid in groups:
         cur.execute("""
-            SELECT f.id, f.width, f.height, f.file_size, f.exif_datetime
+            SELECT f.id, f.path, f.width, f.height, f.file_size,
+                   f.exif_datetime, f.file_mtime
             FROM duplicate_members dm
             JOIN files f ON f.id = dm.file_id
             WHERE dm.group_id = ?