fix(backend): apply auto-tag rule to existing files on creation

CreateRule accepted apply_to_existing but ignored it, so enabling the
checkbox while creating a rule never retroactively tagged files already
carrying the when-tag — only activating an existing rule did. Extract the
retroactive expansion into TagRuleRepo.ApplyToExisting (reused by SetActive)
and call it from CreateRule when the rule is active, inside one transaction
so a file is never left half-tagged. Mirrors SetRuleActive semantics,
including following only active downstream rules.

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

backend/internal/db/postgres/tag_repo.go

@@ -604,10 +604,14 @@ WHERE when_tag_id = $1 AND then_tag_id = $2`
 	if !active || !applyToExisting {
 		return nil
 	}
+	return r.ApplyToExisting(ctx, whenTagID, thenTagID)
+}
 
-	// Retroactively apply the full transitive expansion of thenTagID to all
-	// files that already carry whenTagID. The recursive CTE walks active rules
-	// starting from thenTagID (mirrors the Go expandTagSet BFS).
+// ApplyToExisting retroactively applies the full transitive expansion of
+// thenTagID to all files that already carry whenTagID. The recursive CTE walks
+// active rules starting from thenTagID (mirrors the Go expandTagSet BFS), so
+// inactive downstream rules are not followed. Idempotent via ON CONFLICT.
+func (r *TagRuleRepo) ApplyToExisting(ctx context.Context, whenTagID, thenTagID uuid.UUID) error {
 	const retroQuery = `
 WITH RECURSIVE expansion(tag_id) AS (
     SELECT $2::uuid
@@ -624,8 +628,9 @@ CROSS JOIN expansion e
 WHERE ft.tag_id = $1
 ON CONFLICT DO NOTHING`
 
+	q := connOrTx(ctx, r.pool)
 	if _, err := q.Exec(ctx, retroQuery, whenTagID, thenTagID); err != nil {
-		return fmt.Errorf("TagRuleRepo.SetActive retroactive apply: %w", err)
+		return fmt.Errorf("TagRuleRepo.ApplyToExisting: %w", err)
 	}
 	return nil
 }

backend/internal/integration/server_test.go

@@ -674,6 +674,94 @@ func TestTagRuleActivateApplyToExisting(t *testing.T) {
 	assert.ElementsMatch(t, []string{"animal", "living-thing", "organism"}, tagNames())
 }
 
+// TestTagRuleCreateApplyToExisting verifies that *creating* a rule with
+// apply_to_existing=true retroactively tags existing files — the same contract
+// as activating one (TestTagRuleActivateApplyToExisting), exercised through the
+// POST path. Also checks that apply_to_existing=false and an inactive rule both
+// leave existing files untouched.
+func TestTagRuleCreateApplyToExisting(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+
+	h := setupSuite(t)
+	tok := h.login("admin", "admin")
+
+	mkTag := func(name string) string {
+		resp := h.doJSON("POST", "/tags", map[string]any{"name": name}, tok)
+		require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+		var obj map[string]any
+		resp.decode(t, &obj)
+		return obj["id"].(string)
+	}
+	tagA := mkTag("animal")
+	tagB := mkTag("living-thing")
+	tagC := mkTag("organism")
+
+	// Rule B→C: active, so it fires transitively once B lands on the file.
+	resp := h.doJSON("POST", "/tags/"+tagB+"/rules", map[string]any{
+		"then_tag_id": tagC,
+		"is_active":   true,
+	}, tok)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+
+	// A file carrying only tag A — no A→B rule exists yet.
+	file := h.uploadJPEG(tok, "cat.jpg")
+	fileID := file["id"].(string)
+	resp = h.doJSON("PUT", "/files/"+fileID+"/tags", map[string]any{
+		"tag_ids": []string{tagA},
+	}, tok)
+	require.Equal(t, http.StatusOK, resp.StatusCode, resp.String())
+
+	tagNames := func() []string {
+		r := h.doJSON("GET", "/files/"+fileID+"/tags", nil, tok)
+		require.Equal(t, http.StatusOK, r.StatusCode)
+		var items []any
+		r.decode(t, &items)
+		names := make([]string, 0, len(items))
+		for _, it := range items {
+			names = append(names, it.(map[string]any)["name"].(string))
+		}
+		return names
+	}
+	assert.ElementsMatch(t, []string{"animal"}, tagNames())
+
+	// Creating an INACTIVE rule, even with apply_to_existing=true, must not tag.
+	resp = h.doJSON("POST", "/tags/"+tagA+"/rules", map[string]any{
+		"then_tag_id":       tagB,
+		"is_active":         false,
+		"apply_to_existing": true,
+	}, tok)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+	assert.ElementsMatch(t, []string{"animal"}, tagNames(), "inactive rule must not tag existing files")
+
+	// Drop it so we can recreate as active.
+	resp = h.doJSON("DELETE", "/tags/"+tagA+"/rules/"+tagB, nil, tok)
+	require.Equal(t, http.StatusNoContent, resp.StatusCode, resp.String())
+
+	// Creating an ACTIVE rule with apply_to_existing=false leaves the file alone.
+	resp = h.doJSON("POST", "/tags/"+tagA+"/rules", map[string]any{
+		"then_tag_id":       tagB,
+		"is_active":         true,
+		"apply_to_existing": false,
+	}, tok)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+	assert.ElementsMatch(t, []string{"animal"}, tagNames(), "apply_to_existing=false must not touch existing files")
+
+	resp = h.doJSON("DELETE", "/tags/"+tagA+"/rules/"+tagB, nil, tok)
+	require.Equal(t, http.StatusNoContent, resp.StatusCode, resp.String())
+
+	// Creating A→B active WITH apply_to_existing=true: the file gets B directly
+	// and C transitively via the already-active B→C rule.
+	resp = h.doJSON("POST", "/tags/"+tagA+"/rules", map[string]any{
+		"then_tag_id":       tagB,
+		"is_active":         true,
+		"apply_to_existing": true,
+	}, tok)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+	assert.ElementsMatch(t, []string{"animal", "living-thing", "organism"}, tagNames())
+}
+
 // TestTagAutoRule verifies that adding a tag automatically applies then_tags.
 func TestTagAutoRule(t *testing.T) {
 	if testing.Short() {

backend/internal/port/repository.go

@@ -101,6 +101,10 @@ type TagRuleRepo interface {
 	// 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
+	// ApplyToExisting retroactively applies the full transitive expansion of
+	// thenTagID (following active rules) to every file that already carries
+	// whenTagID. Used when a rule is created or activated with apply_to_existing.
+	ApplyToExisting(ctx context.Context, whenTagID, thenTagID uuid.UUID) error
 	Delete(ctx context.Context, whenTagID, thenTagID uuid.UUID) error
 }
 

backend/internal/service/tag_service.go

@@ -192,16 +192,32 @@ func (s *TagService) ListRules(ctx context.Context, tagID uuid.UUID) ([]domain.T
 	return s.rules.ListByTag(ctx, tagID)
 }
 
-// CreateRule adds a tag rule. If applyToExisting is true, the then_tag is
-// retroactively applied to all files that already carry the when_tag.
-// Retroactive application requires a FileRepo; it is deferred until wired
-// in a future iteration (see port.FileRepo.ListByTag).
-func (s *TagService) CreateRule(ctx context.Context, whenTagID, thenTagID uuid.UUID, isActive, _ bool) (*domain.TagRule, error) {
-	return s.rules.Create(ctx, domain.TagRule{
+// CreateRule adds a tag rule. When the rule is active and applyToExisting is
+// true, the full transitive expansion of thenTagID is retroactively applied to
+// every file already carrying whenTagID — same semantics as activating an
+// existing rule via SetRuleActive. The insert and retroactive apply run in one
+// transaction so a file is never left half-tagged.
+func (s *TagService) CreateRule(ctx context.Context, whenTagID, thenTagID uuid.UUID, isActive, applyToExisting bool) (*domain.TagRule, error) {
+	var created *domain.TagRule
+	err := s.tx.WithTx(ctx, func(ctx context.Context) error {
+		rule, err := s.rules.Create(ctx, domain.TagRule{
 			WhenTagID: whenTagID,
 			ThenTagID: thenTagID,
 			IsActive:  isActive,
 		})
+		if err != nil {
+			return err
+		}
+		created = rule
+		if isActive && applyToExisting {
+			return s.rules.ApplyToExisting(ctx, whenTagID, thenTagID)
+		}
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return created, nil
 }
 
 // SetRuleActive toggles a rule's is_active flag and returns the updated rule.