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>

apps/broker/src/crypto.ts

@@ -10,7 +10,7 @@
 import { and, eq, isNull, lt, sql } from "drizzle-orm";
 import sodium from "libsodium-wrappers";
 import { db } from "./db";
-import { invite as inviteTable, mesh, meshMember } from "@turbostarter/db/schema/mesh";
+import { invite as inviteTable, mesh, meshMember, meshTopic, meshTopicMember } from "@turbostarter/db/schema/mesh";
 
 let ready = false;
 async function ensureSodium(): Promise<typeof sodium> {
@@ -344,6 +344,32 @@ export async function claimInviteV2Core(params: {
     return { ok: false, status: 400, body: { error: "malformed" } };
   }
 
+  // 6b. Auto-subscribe the new member to #general (the default mesh-wide
+  // room). Idempotent via unique (topic_id, member_id). If the mesh was
+  // created before #general auto-creation existed, ensure it now via a
+  // best-effort INSERT … ON CONFLICT — backfill migration handles the
+  // bulk case so this is just a safety net.
+  await db
+    .insert(meshTopic)
+    .values({
+      meshId: inv.meshId,
+      name: "general",
+      description: "Default mesh-wide channel. Every member can read and post.",
+      visibility: "public",
+    })
+    .onConflictDoNothing();
+  const [generalTopic] = await db
+    .select({ id: meshTopic.id })
+    .from(meshTopic)
+    .where(and(eq(meshTopic.meshId, inv.meshId), eq(meshTopic.name, "general")))
+    .limit(1);
+  if (generalTopic) {
+    await db
+      .insert(meshTopicMember)
+      .values({ topicId: generalTopic.id, memberId: row.id, role: "member" })
+      .onConflictDoNothing();
+  }
+
   // 7. Seal the mesh root_key to the recipient's x25519 pubkey.
   let sealed: string;
   try {