feat(notify): bundled notifications, task watchers, daily summary

- Per-user routing (own mail / own Discord webhook) for task alarms
  (sent at once), shares, completed shared tasks and watched tasks;
  settings as JSON on the user (GET /auth/me, PATCH /auth/settings).
- Everything but alarms is queued and sent as one message per user
  after NOTIFY_DIGEST_MINUTES (default 5); mails now multipart with an
  HTML version, Discord as one embed. Three failed attempts drop it.
- Own changes never notify the actor, except on watched tasks when
  asked for; "paused" stops everything including alarms.
- Task watchers: POST/DELETE /tasks/{id}/watch, isWatching on tasks.
- Daily summary at a chosen time: overdue, today, tomorrow, plus the
  count of the next 7 days; nothing due, nothing sent.
- Migration 7; jobs do not run in the demo.

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

internal/api/handlers_documents.go

@@ -262,6 +262,9 @@ func (s *Server) handleCreateDocShare(w http.ResponseWriter, r *http.Request) {
 
 	if req.TeamID != "" {
 		share, err := s.store.CreateDocTeamShare(userID, docID, req.TeamID, req.Permission)
+		if err == nil {
+			defer s.notifyShared(userID, "team", req.TeamID, "die Notiz", s.docTitle(userID, docID))
+		}
 		if err != nil {
 			writeError(w, http.StatusBadRequest, err.Error())
 			return
@@ -286,6 +289,7 @@ func (s *Server) handleCreateDocShare(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, err.Error())
 		return
 	}
+	s.notifyShared(userID, "user", share.UserID, "die Notiz", s.docTitle(userID, docID))
 	s.broadcastDocToAuthorizedUsers(docID, "DOC_CREATED")
 	s.broadcastDocToAuthorizedUsers(docID, "DOC_UPDATED")
 	writeJSON(w, http.StatusCreated, share)
@@ -313,3 +317,10 @@ func (s *Server) handleGetCollaborators(w http.ResponseWriter, r *http.Request)
 	}
 	writeJSON(w, http.StatusOK, users)
 }
+
+func (s *Server) docTitle(userID, docID string) string {
+	if d, err := s.store.GetDocumentByID(userID, docID); err == nil && d.Title != "" {
+		return d.Title
+	}
+	return "Unbenannte Notiz"
+}