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/migrations/003_data_tables.sql

@@ -55,7 +55,8 @@ CREATE TABLE data.files (
     creator_id       smallint     NOT NULL REFERENCES core.users(id)
                                   ON UPDATE CASCADE ON DELETE RESTRICT,
     is_public        boolean      NOT NULL DEFAULT false,
-    is_deleted       boolean      NOT NULL DEFAULT false  -- soft delete (trash)
+    is_deleted       boolean      NOT NULL DEFAULT false, -- soft delete (trash)
+    needs_review     boolean      NOT NULL DEFAULT true   -- tagging not yet marked done; cleared explicitly
 );
 
 CREATE TABLE data.file_tag (