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/007_seed_data.sql

@@ -20,7 +20,7 @@ INSERT INTO activity.action_types (name) VALUES
     ('user_login'), ('user_logout'),
     -- Files
     ('file_create'), ('file_edit'), ('file_delete'), ('file_restore'),
-    ('file_permanent_delete'), ('file_replace'),
+    ('file_permanent_delete'), ('file_replace'), ('file_review'),
     -- Tags
     ('tag_create'), ('tag_edit'), ('tag_delete'),
     -- Categories