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/config/config.go

@@ -51,6 +51,9 @@ type Config struct {
 
 	// Images and audio in notes (MEDIA_*)
 	Media MediaConfig
+
+	// NotifyDigestMinutes: notifications (except task alarms) are bundled over this many minutes
+	NotifyDigestMinutes int
 }
 
 // MediaConfig limits uploads of images and audio recordings in notes. Sizes in MB; QuotaMB 0 = no limit
@@ -237,6 +240,7 @@ func Load() *Config {
 			PerUserPerHour: envInt("STT_PER_USER_PER_HOUR", 60, 1, 100000),
 			TimeoutSeconds: envInt("STT_TIMEOUT_SECONDS", 120, 5, 3600),
 		},
+		NotifyDigestMinutes: envInt("NOTIFY_DIGEST_MINUTES", 5, 1, 1440),
 		Media: MediaConfig{
 			MaxImageMB: envInt("MEDIA_MAX_IMAGE_MB", 15, 1, 1000),
 			MaxAudioMB: envInt("MEDIA_MAX_AUDIO_MB", 50, 1, 2000),