feat(phase 3d auto-tag): backfill area tags, bulk-edit UI, soft-delete cleanup

- migration 0012: one-shot populate empty tags from each item's area-roots
  (so chips on /?tag=work etc. actually filter the 40+ mai-backfilled rows)
- migration 0013: cleanup 12 orphan item_links + BEFORE-UPDATE trigger that
  cascades soft-delete to item_links going forward — closes the data drift
  that made TestItemsUnifiedSurfacesMaiPointer fail since 3c
- /admin/bulk page: flat filter+checkbox list with one-tx Apply for add/
  remove tag, set management, set status. Per-row inline chip add/remove
  via /admin/bulk/chip. Reuses tree_filter URL params 1:1.
- design.md §3.2 + §4.1 updated; tag+management section notes 0012
- bulk + tag-backfill + soft-delete-cascade tests cover the new surface

web/server.go

@@ -133,6 +133,34 @@ func New(s *store.Store, logger *slog.Logger) (*Server, error) {
 		return nil, fmt.Errorf("parse login: %w", err)
 	}
 	pages["login"] = loginTmpl
+
+	// Bulk-edit page + its fragment + per-row chip cells. The chip cells share
+	// definitions with bulk_section so we parse them together every time.
+	bulkTmpl, err := template.New("bulk").Funcs(funcs).ParseFS(templatesFS,
+		"templates/layout.tmpl",
+		"templates/bulk.tmpl",
+		"templates/bulk_section.tmpl",
+	)
+	if err != nil {
+		return nil, fmt.Errorf("parse bulk: %w", err)
+	}
+	pages["bulk"] = bulkTmpl
+	bulkSection, err := template.New("bulk_section").Funcs(funcs).ParseFS(templatesFS, "templates/bulk_section.tmpl")
+	if err != nil {
+		return nil, fmt.Errorf("parse bulk_section: %w", err)
+	}
+	pages["bulk_section"] = bulkSection
+	bulkChipTags, err := template.New("bulk_chip_tags").Funcs(funcs).ParseFS(templatesFS, "templates/bulk_section.tmpl")
+	if err != nil {
+		return nil, fmt.Errorf("parse bulk_chip_tags: %w", err)
+	}
+	pages["bulk_chip_tags"] = bulkChipTags
+	bulkChipMgmt, err := template.New("bulk_chip_mgmt").Funcs(funcs).ParseFS(templatesFS, "templates/bulk_section.tmpl")
+	if err != nil {
+		return nil, fmt.Errorf("parse bulk_chip_mgmt: %w", err)
+	}
+	pages["bulk_chip_mgmt"] = bulkChipMgmt
+
 	return &Server{Store: s, pages: pages, Logger: logger}, nil
 }
 
@@ -146,6 +174,9 @@ func (s *Server) Routes() http.Handler {
 	mux.HandleFunc("GET /new", s.handleNewForm)
 	mux.HandleFunc("POST /new", s.handleNewSubmit)
 	mux.HandleFunc("GET /admin/classify", s.handleClassify)
+	mux.HandleFunc("GET /admin/bulk", s.handleBulk)
+	mux.HandleFunc("POST /admin/bulk/apply", s.handleBulkApply)
+	mux.HandleFunc("POST /admin/bulk/chip", s.handleBulkChip)
 	mux.HandleFunc("GET /admin/caldav", s.handleCalDAVAdmin)
 	mux.HandleFunc("POST /admin/caldav/link", s.handleCalDAVLink)
 	mux.HandleFunc("POST /admin/caldav/unlink", s.handleCalDAVUnlink)
@@ -569,6 +600,8 @@ func (s *Server) render(w http.ResponseWriter, name string, data map[string]any)
 		entry = "tree-section"
 	case "documents_section":
 		entry = "documents-section"
+	case "bulk_section":
+		entry = "bulk-section"
 	}
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	if err := t.ExecuteTemplate(w, entry, data); err != nil {