feat: per-pool automatic file sorting

Pools now store a sort setting (sort_key + sort_order). "manual" keeps the
user-arranged order in file_pool.position and allows drag-to-reorder; any
other key (content_datetime, created, original_name) sorts the pool's files
automatically server-side, in which case reordering is rejected. Manual
order is always ascending by position — direction does not apply.

Backend: add the columns (sort_key defaults to 'manual'), generalise the
pool-files keyset cursor to page by the active sort, persist the setting on
create/update, and guard reorder against non-manual pools.

Frontend: a sort dropdown (+ direction toggle) on the pool page that PATCHes
the pool and reloads; drag-to-reorder is disabled unless the pool is manual.

Closes the "automatic sorting in pools" requirement (REQUIREMENTS.md §4.6.5).

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

openapi.yaml

@@ -2281,6 +2281,16 @@ components:
           type: string
         is_public:
           type: boolean
+        sort_key:
+          type: string
+          enum: [manual, content_datetime, created, original_name]
+          description: >-
+            How the pool's files are ordered. "manual" keeps the user-defined
+            order (drag-to-reorder); any other key sorts the files automatically
+            by that file field and disables manual reordering.
+        sort_order:
+          type: string
+          enum: [asc, desc]
         file_count:
           type: integer
         created_at:
@@ -2312,6 +2322,12 @@ components:
           type: object
         is_public:
           type: boolean
+        sort_key:
+          type: string
+          enum: [manual, content_datetime, created, original_name]
+        sort_order:
+          type: string
+          enum: [asc, desc]
 
     PoolOffsetPage:
       type: object