feat(maintenance): schema for reminders dedup log

Phase 1 of the maintenance-reminders feature.

- notification_kind: + maintenance_due_soon, maintenance_overdue.
  These are distinct from maintenance_event_recorded (service
  performed) so the bell list can group/filter reminder vs service
  cleanly.
- New maintenance_reminder_kind enum: due_soon | overdue.
- New maintenance_reminders_sent table with UNIQUE(schedule_id,
  kind, due_at). The cron uses INSERT … ON CONFLICT DO NOTHING on
  this composite to make the fire-once-per-window guarantee atomic
  even under concurrent runs. Once the schedule's next_due_at
  advances after a service event, the tuple is fresh and a new
  reminder fires.

No service code yet — Phase 2 wires the readers and orchestrator.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

drizzle/meta/_journal.json

@@ -134,6 +134,13 @@
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       "tag": "0018_checklist_scope_property",
       "breakpoints": true
+    },
+    {
+      "idx": 19,
+      "version": "7",
+      "when": 1777281036233,
+      "tag": "0019_maintenance_reminders",
+      "breakpoints": true
     }
   ]
 }