feat(notes): operation log, block leases and sync API for collaborative editing

Phase 1 of the collaboration plan (server side):
- internal/collab: block tree with parent IDs and fractional order keys,
  operations with conflict rules (version check, block leases, conflict
  copies instead of lost text, restore on late edits, cycle checks),
  in-memory leases/sessions and long-poll notifier.
- Storage: per-note lock + transaction, append-only effect log (doc_ops),
  idempotent results per opId (doc_op_results), snapshot, log paging,
  central DocRole. Old flat block data is converted to the tree.
- The existing whole-document save (PUT/PATCH) runs as an adapter through
  the same engine, so the current editor keeps working and writes to the log.
- API: GET /docs/{id}/snapshot, POST /docs/{id}/sync, GET /docs/{id}/changes
  (long poll), POST /docs/{id}/leave; DOC_SEQ poke for WebSocket/SSE rooms.
- Backend accepts editor (write) shares for notes and folders; the forced
  read-only migration is gone. The share dialog still offers read only.
- Tests: conflict matrix, order keys, leases, storage adapter round trip,
  idempotency, API protocol incl. long poll, and a randomized simulation
  (4 clients, lost/duplicated requests and responses, offline phases,
  lease expiry) checking convergence, no lost text and exactly-once results.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -18,6 +18,7 @@ import (
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/go-chi/cors"
 	"github.com/sandrodach/mindsift/internal/auth"
+	"github.com/sandrodach/mindsift/internal/collab"
 	"github.com/sandrodach/mindsift/internal/config"
 	"github.com/sandrodach/mindsift/internal/domain"
 	"github.com/sandrodach/mindsift/internal/storage"
@@ -29,6 +30,9 @@ type Server struct {
 	store    *storage.Store
 	wsHub    *ws.Hub
 	staticFS fs.FS
+
+	leases   *collab.LeaseManager // block leases and open sessions of notes
+	notifier *collab.Notifier     // wakes long-poll requests on note changes
 }
 
 func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS fs.FS) *Server {
@@ -37,6 +41,8 @@ func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS
 		store:    store,
 		wsHub:    wsHub,
 		staticFS: staticFS,
+		leases:   collab.NewLeaseManager(nil),
+		notifier: collab.NewNotifier(),
 	}
 	wsHub.SetRoomAuthorizer(srv.canJoinRoom)
 	go srv.startAlarmTicker()
@@ -46,8 +52,7 @@ func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS
 // canJoinRoom allows realtime rooms only to users with read access: "doc:<id>" for notes, otherwise a shared list ID.
 func (s *Server) canJoinRoom(userID, room string) bool {
 	if docID, ok := strings.CutPrefix(room, "doc:"); ok {
-		_, err := s.store.GetDocumentByID(userID, docID)
-		return err == nil
+		return s.store.DocRole(userID, docID) != ""
 	}
 	_, err := s.store.GetListByID(userID, room)
 	return err == nil
@@ -159,6 +164,12 @@ func (s *Server) Router() http.Handler {
 		r.Delete("/api/v1/documents/{id}", s.handleDeleteDocument)
 		r.Post("/api/v1/documents/{id}/blocks/{blockId}/enhanced-task", s.handleCreateEnhancedTask)
 
+		// Collaborative note editing (operation log)
+		r.Get("/api/v1/docs/{id}/snapshot", s.handleDocSnapshot)
+		r.Post("/api/v1/docs/{id}/sync", s.handleDocSync)
+		r.Get("/api/v1/docs/{id}/changes", s.handleDocChanges)
+		r.Post("/api/v1/docs/{id}/leave", s.handleDocLeave)
+
 		// Document Sharing & Collaboration
 		r.Get("/api/v1/documents/{id}/shares", s.handleGetDocShares)
 		r.Post("/api/v1/documents/{id}/shares", s.handleCreateDocShare)