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/handler/pool_handler.go

@@ -34,6 +34,8 @@ type poolJSON struct {
 	CreatorID   int16   `json:"creator_id"`
 	CreatorName string  `json:"creator_name"`
 	IsPublic    bool    `json:"is_public"`
+	SortKey     string  `json:"sort_key"`
+	SortOrder   string  `json:"sort_order"`
 	FileCount   int     `json:"file_count"`
 	CreatedAt   string  `json:"created_at"`
 }
@@ -51,6 +53,8 @@ func toPoolJSON(p domain.Pool) poolJSON {
 		CreatorID:   p.CreatorID,
 		CreatorName: p.CreatorName,
 		IsPublic:    p.IsPublic,
+		SortKey:     p.SortKey,
+		SortOrder:   p.SortOrder,
 		FileCount:   p.FileCount,
 		CreatedAt:   p.CreatedAt.UTC().Format(time.RFC3339),
 	}
@@ -214,6 +218,16 @@ func (h *PoolHandler) Update(c *gin.Context) {
 			params.IsPublic = &b
 		}
 	}
+	if v, ok := raw["sort_key"]; ok {
+		if s, ok := v.(string); ok {
+			params.SortKey = &s
+		}
+	}
+	if v, ok := raw["sort_order"]; ok {
+		if s, ok := v.(string); ok {
+			params.SortOrder = &s
+		}
+	}
 
 	updated, err := h.poolSvc.Update(c.Request.Context(), id, params)
 	if err != nil {