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/domain/pool.go

@@ -7,6 +7,32 @@ import (
 	"github.com/google/uuid"
 )
 
+// Pool file sort keys. PoolSortManual keeps the user-defined order in
+// file_pool.position; the others sort the pool's files by that file field.
+const (
+	PoolSortManual          = "manual"
+	PoolSortContentDatetime = "content_datetime"
+	PoolSortCreated         = "created"
+	PoolSortOriginalName    = "original_name"
+
+	SortOrderAsc  = "asc"
+	SortOrderDesc = "desc"
+)
+
+// ValidPoolSortKey reports whether s is an accepted pool sort key.
+func ValidPoolSortKey(s string) bool {
+	switch s {
+	case PoolSortManual, PoolSortContentDatetime, PoolSortCreated, PoolSortOriginalName:
+		return true
+	}
+	return false
+}
+
+// ValidSortOrder reports whether s is an accepted sort direction.
+func ValidSortOrder(s string) bool {
+	return s == SortOrderAsc || s == SortOrderDesc
+}
+
 // Pool is an ordered collection of files.
 type Pool struct {
 	ID          uuid.UUID
@@ -16,6 +42,11 @@ type Pool struct {
 	CreatorID   int16
 	CreatorName string // denormalized
 	IsPublic    bool
+	// SortKey / SortOrder control how the pool's files are ordered. When SortKey
+	// is PoolSortManual, files follow the manual position order and can be
+	// reordered; otherwise they are sorted automatically and reordering is a no-op.
+	SortKey     string
+	SortOrder   string
 	FileCount   int
 	CreatedAt   time.Time // extracted from UUID v7 via UUIDCreatedAt
 }