feat(backend): per-file review status with DSL filter and bulk endpoint

Replaces the old "untagged" sentinel tag with a proper per-file workflow
status: needs_review starts true on upload/import and is cleared by an
explicit action (no auto-clear on tagging). Surfaced as a filter token
(r=1 needs review, r=0 done) so it combines with tag/MIME conditions, and
toggled via POST /files/bulk/review (single id or many, edit-ACL enforced,
audit-logged as file_review).

needs_review lives on data.files (column added to the original 003 migration,
partial index in 006, action type seeded in 007).

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

backend/internal/handler/router.go

@@ -83,6 +83,7 @@ func NewRouter(
 		// Bulk + import routes registered before /:id to prevent param collision.
 		files.POST("/bulk/tags", fileHandler.BulkSetTags)
 		files.POST("/bulk/delete", fileHandler.BulkDelete)
+		files.POST("/bulk/review", fileHandler.BulkReview)
 		files.POST("/bulk/common-tags", fileHandler.CommonTags)
 		files.POST("/import", fileHandler.Import)