* files: SQL FileGroup compare, stem column, drop unused discovery path
Refresh walked the tree twice (find, then stat every unchanged group),
and a two-directory refresh scanned the whole media directory. Compare
now diffs in SQL on a TEMP table that already has mtime, accepts multiple
roots, and groups by a stored stem that is versioned so a suffix-rule
change cannot delete-and-reinsert tagged files.
Deletes refresh_discover_paths and /api/files/refresh_progress, which
nothing in production called. Video/doc/zim modelers use the same
(no domain row OR indexed != True) gate.
* files: keep stem current on reorganize; NUL-safe find parse
Reorganize wrote the new primary_path but not stem, so the next refresh
treated the FileGroup as deleted+new and dropped tags. GNU find now
emits NUL-delimited records so a tab or newline in a filename cannot
abort a global refresh. Stem backfill skips the table scan once the
algorithm version matches, and runs off the event loop. An empty roots
list is a global refresh, not invalid SQL.