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>

backend/internal/port/repository.go

@@ -83,8 +83,10 @@ type TagRuleRepo interface {
 	// ListByTag returns all rules where WhenTagID == tagID.
 	ListByTag(ctx context.Context, tagID uuid.UUID) ([]domain.TagRule, error)
 	Create(ctx context.Context, r domain.TagRule) (*domain.TagRule, error)
-	// SetActive toggles a rule's is_active flag.
-	SetActive(ctx context.Context, whenTagID, thenTagID uuid.UUID, active bool) error
+	// SetActive toggles a rule's is_active flag. When active and applyToExisting
+	// are both true, the full transitive expansion of thenTagID is retroactively
+	// applied to all files that already carry whenTagID.
+	SetActive(ctx context.Context, whenTagID, thenTagID uuid.UUID, active, applyToExisting bool) error
 	Delete(ctx context.Context, whenTagID, thenTagID uuid.UUID) error
 }