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/0020_sla_tables.sql

@@ -0,0 +1,15 @@
+CREATE TABLE sla_policies (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  queue_id UUID REFERENCES queues(id) ON DELETE SET NULL,
+  name TEXT NOT NULL,
+  description TEXT,
+  response_time_minutes INTEGER,
+  resolution_time_minutes INTEGER,
+  disabled BOOLEAN NOT NULL DEFAULT false,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+CREATE INDEX sla_policies_queue_id_idx ON sla_policies(queue_id);
+
+ALTER TABLE tickets ADD COLUMN sla_response_deadline TIMESTAMPTZ;
+ALTER TABLE tickets ADD COLUMN sla_resolution_deadline TIMESTAMPTZ;
+ALTER TABLE tickets ADD COLUMN sla_breached TEXT;