feat(backend): apply tag rules retroactively to existing files on activation

Extend PATCH /tags/{id}/rules/{then_id} to accept apply_to_existing bool.
When a rule is activated with apply_to_existing=true, a single recursive
CTE retroactively inserts the full transitive expansion of then_tag into
data.file_tag for all files already carrying when_tag:

  WITH RECURSIVE expansion(tag_id) AS (
      SELECT then_tag_id
      UNION
      SELECT r.then_tag_id FROM data.tag_rules r
      JOIN expansion e ON r.when_tag_id = e.tag_id
      WHERE r.is_active = true
  )
  INSERT INTO data.file_tag ... ON CONFLICT DO NOTHING

Changes:
- port/repository.go: add applyToExisting param to TagRuleRepo.SetActive
- db/postgres/tag_repo.go: implement recursive CTE retroactive apply
- service/tag_service.go: thread applyToExisting through SetRuleActive
- handler/tag_handler.go: parse apply_to_existing from PATCH body
- openapi.yaml: document apply_to_existing on PATCH endpoint
- integration test: add TestTagRuleActivateApplyToExisting covering
  no-op when false, direct+transitive apply when true

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

openapi.yaml

@@ -846,6 +846,10 @@ paths:
               properties:
                 is_active:
                   type: boolean
+                apply_to_existing:
+                  type: boolean
+                  default: false
+                  description: When activating, apply rule retroactively to files already tagged
       responses:
         '200':
           description: Rule updated