feat(heartbeat): add configurable visibility for heartbeat responses

Add per-channel and per-account heartbeat visibility settings:
- showOk: hide/show HEARTBEAT_OK messages (default: false)
- showAlerts: hide/show alert messages (default: true)
- useIndicator: emit typing indicator events (default: true)

Config precedence: per-account > per-channel > channel-defaults > global

This allows silencing routine heartbeat acks while still surfacing
alerts when something needs attention.

src/config/zod-schema.channels.ts

@@ -0,0 +1,10 @@
+import { z } from "zod";
+
+export const ChannelHeartbeatVisibilitySchema = z
+  .object({
+    showOk: z.boolean().optional(),
+    showAlerts: z.boolean().optional(),
+    useIndicator: z.boolean().optional(),
+  })
+  .strict()
+  .optional();