feat: watcher/CC system, SLA engine, and rich text comments

- Watcher system: ticket_watchers table, watch/unwatch endpoints,
  notifications to watchers on comments and updates, watcher/cc
  recipient sources in SendEmail scrip action, watch toggle and
  watcher avatars in ticket detail UI
- SLA engine: sla_policies table, SLA deadline columns on tickets,
  CRUD routes, OnSlaBreach scrip condition, scheduler SLA calculation,
  deadlines set on create/reply, cleared on resolve, SLA indicators
  on ticket list and detail, SLA Policies tab in admin
- Rich text: marked-based markdown rendering with XSS safety,
  Write/Preview toggle in comment composer, styled prose output

drizzle/migrations/meta/_journal.json

@@ -134,6 +134,20 @@
       "when": 1781551130161,
       "tag": "0018_dapper_jack_power",
       "breakpoints": true
+    },
+    {
+      "idx": 19,
+      "version": "7",
+      "when": 1781552000000,
+      "tag": "0019_watcher_tables",
+      "breakpoints": true
+    },
+    {
+      "idx": 20,
+      "version": "7",
+      "when": 1781552001000,
+      "tag": "0020_sla_tables",
+      "breakpoints": true
     }
   ]
 }