feat(frontend): admin "Import from server" panel in Settings

Surfaces the previously UI-less POST /files/import: an admin-only Settings
card with an optional subfolder field, an Import button, and a result
summary (imported / skipped / per-file errors). Notes that imported files
are drained from the folder and that mtime is kept as the date when EXIF
is absent. Also documents the endpoint's drain + mtime behaviour in the
OpenAPI spec.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

openapi.yaml

@@ -648,6 +648,12 @@ paths:
     post:
       tags: [Files]
       summary: Import files from a server directory
+      description: >
+        Admin only. Ingests supported files from the server's configured import
+        directory (optionally a subfolder of it). Subdirectories are skipped and
+        not recursed. A successfully imported file is removed from the import
+        folder. For files without an EXIF date, the source file's modified time
+        is used as content_datetime.
       requestBody:
         required: true
         content: