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>

backend/internal/port/repository.go

@@ -36,6 +36,10 @@ type PoolFileListParams struct {
 	Cursor string
 	Limit  int
 	Filter string // filter DSL expression
+	// SortKey / SortOrder come from the pool itself (not the request) and select
+	// how files are ordered: manual position order or an automatic file-field sort.
+	SortKey   string
+	SortOrder string
 }
 
 // FileRepo is the persistence interface for file records.