feat(broker+api): every mesh ships with a default #general topic

The web chat surface needed a guaranteed landing room — a topic that
exists for every mesh from creation onward so the dashboard always has
somewhere to drop the user. #general is the convention; ephemeral DMs
remain ephemeral (mesh.message_queue) so agentic privacy is unchanged.

Three hooks plus a backfill:

- packages/api/src/modules/mesh/mutations.ts — createMyMesh now calls
  ensureGeneralTopic() right after the mesh insert. New helper is
  idempotent via the unique (mesh_id, name) index.
- apps/broker/src/index.ts — handleMeshCreate (CLI claudemesh new)
  inserts #general + subscribes the owner member as 'lead' in the
  same handler.
- apps/broker/src/crypto.ts — invite-claim flow auto-subscribes the
  newly minted member to #general as 'member', defensively ensuring
  the topic exists if predates this change.
- packages/db/migrations/0024_general_topic_backfill.sql — one-shot
  backfill: creates #general for every active mesh that doesn't have
  one, subscribes every active member, and marks the mesh owner as
  'lead' based on owner_user_id == member.user_id. Idempotent.

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

packages/api/src/modules/mesh/mutations.ts

@@ -3,7 +3,14 @@ import { randomBytes } from "node:crypto";
 import sodium from "libsodium-wrappers";
 
 import { and, eq, isNull } from "@turbostarter/db";
-import { invite, mesh, meshMember, pendingInvite } from "@turbostarter/db/schema";
+import {
+  invite,
+  mesh,
+  meshMember,
+  meshTopic,
+  meshTopicMember,
+  pendingInvite,
+} from "@turbostarter/db/schema";
 import { db } from "@turbostarter/db/server";
 
 import type {
@@ -142,9 +149,47 @@ export const createMyMesh = async ({
     })
     .returning({ id: mesh.id, slug: mesh.slug });
 
+  if (created) {
+    await ensureGeneralTopic(created.id);
+  }
+
   return created!;
 };
 
+/**
+ * Idempotently create the conventional `#general` topic for a mesh.
+ *
+ * `#general` is the default web-readable room: a public topic that every
+ * mesh has so the dashboard chat surface always has somewhere to land.
+ * Subscription is not required for read access via the REST surface, so
+ * subscribing members happens lazily at member-row creation time
+ * (invite-claim) rather than here.
+ *
+ * Safe to call repeatedly — the unique (meshId, name) index keeps it a
+ * no-op on the second call.
+ */
+export const ensureGeneralTopic = async (
+  meshId: string,
+): Promise<{ id: string } | null> => {
+  const [existing] = await db
+    .select({ id: meshTopic.id })
+    .from(meshTopic)
+    .where(and(eq(meshTopic.meshId, meshId), eq(meshTopic.name, "general")))
+    .limit(1);
+  if (existing) return existing;
+  const [row] = await db
+    .insert(meshTopic)
+    .values({
+      meshId,
+      name: "general",
+      description: "Default mesh-wide channel. Every member can read and post.",
+      visibility: "public",
+    })
+    .onConflictDoNothing()
+    .returning({ id: meshTopic.id });
+  return row ?? null;
+};
+
 export const archiveMyMesh = async ({
   userId,
   meshId,