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>

Doku/api/websocket.md

@@ -43,6 +43,7 @@ Client → Server versteht der Server nur: `{"event":"JOIN_ROOM","room":"<id>"}`
 | `TASK_REORDERED` | Aufgaben umsortieren | Benutzer aller betroffenen Listen |
 | `TASK_LIST_UPDATED` / `TASK_LIST_DELETED` | Liste ändern, (un)teilen, löschen; Payload nur `{id}`, Clients laden ihre Listen neu | Benutzer vor und nach der Änderung |
 | `DOC_CREATED`, `DOC_UPDATED`, `DOC_DELETED` | Notiz anlegen/ändern/löschen, Freigaben | berechtigte Benutzer |
+| `DOC_SEQ` | Änderung an Notiz, Leases oder Präsenz (`docId`, `headSeq`); Clients holen die Daten per `/docs/{id}/sync` | Raum `doc:<id>` |
 | `DOC_BLOCKS_PATCHED` | `PATCH /documents/{id}/blocks` (`docId`, `updatedBy`, `upserted`, `deletedIds`, `blockOrder`) | Raum `doc:<id>` und berechtigte Benutzer |
 | `WORKSPACE_FOLDERS_UPDATED` | Ordner/Mounts ändern | Benutzer selbst; bei geteilten Ordnern Eigentümer und alle Beteiligten |
 | `LIST_CREATED`, `LIST_UPDATED`, `LIST_DELETED`, `LIST_SHARED`, `LIST_UNSHARED`, `LIST_CLEARED` | Listenverwaltung | Benutzer bzw. Raum der Liste |