fix(backend): enforce private-by-default visibility and pool-op ACL

Listings returned every row regardless of ownership: GET /files, /tags,
/pools and /categories exposed other users' private items (while the
single-item GET correctly returned 403), and the pool file operations
(GET /pools/:id, /pools/:id/files, add/remove/reorder) skipped ACL
entirely, so any authenticated user could read and rewrite anyone's
private pool.

- List queries now filter to rows the caller may see (public, owned, or
  granted can_view) via a shared SQL condition; admins bypass. The viewer
  identity is taken from the request context by the service and passed to
  the repository in the list params.
- Tag/Category/Pool single-item Get now enforce CanView (File already did).
- Pool Get/ListFiles require pool view; AddFiles/RemoveFiles/Reorder
  require pool edit.

Adds regression tests for private-by-default listing (hidden / public /
granted / admin) and for pool operations rejecting a non-owner.

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

backend/internal/domain/file.go

@@ -49,6 +49,12 @@ type FileListParams struct {
 	Filter string // filter DSL expression
 	Search string // substring match on original_name
 	Trash  bool   // if true, return only soft-deleted files
+
+	// Visibility — populated by the service from the request context. When
+	// ViewerIsAdmin is false the repository restricts results to files the
+	// viewer may see (public, owned, or explicitly granted).
+	ViewerID      int16
+	ViewerIsAdmin bool
 }
 
 // FilePage is the result of a cursor-based file listing.