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_thoughts.go

@@ -94,6 +94,7 @@ func (s *Server) handleGetThoughts(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return
 	}
+	s.markWatching(userID, thoughts)
 	writeJSON(w, http.StatusOK, thoughts)
 }
 
@@ -278,6 +279,8 @@ func (s *Server) handleUpdateThought(w http.ResponseWriter, r *http.Request) {
 		s.notifyDocChanged(docID, head)
 	}
 	s.wsHub.BroadcastToUsers(append(s.taskAudience(before), s.taskAudience(updated)...), "TASK_UPDATED", updated)
+	s.notifyTaskChange(userID, before, updated, false)
+	updated.IsWatching = s.store.WatchedTaskIDs(userID)[updated.ID]
 	writeJSON(w, http.StatusOK, updated)
 }
 
@@ -304,6 +307,9 @@ func (s *Server) handleDeleteThought(w http.ResponseWriter, r *http.Request) {
 		writeStoreError(w, http.StatusInternalServerError, err)
 		return
 	}
+	if th != nil {
+		s.notifyTaskChange(userID, th, nil, true)
+	}
 	s.wsHub.BroadcastToUsers(audience, "TASK_DELETED", map[string]string{"id": thoughtID})
 	writeJSON(w, http.StatusOK, map[string]bool{"deleted": true})
 }