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/db/migrations/0024_general_topic_backfill.sql

@@ -0,0 +1,34 @@
+-- 0024_general_topic_backfill.sql
+--
+-- Every mesh now ships with a default #general topic auto-created on mesh
+-- creation. This migration backfills the convention for meshes that
+-- predate that hook:
+--   1. Insert a #general row for every mesh that doesn't already have one.
+--   2. Subscribe every active (non-revoked) member to #general.
+--
+-- Idempotent — safe to re-run. The unique indices on (mesh_id, name) and
+-- (topic_id, member_id) make the inserts no-ops on the second pass.
+
+INSERT INTO mesh.topic (mesh_id, name, description, visibility)
+SELECT
+  m.id,
+  'general',
+  'Default mesh-wide channel. Every member can read and post.',
+  'public'
+FROM mesh.mesh m
+WHERE m.archived_at IS NULL
+  AND NOT EXISTS (
+    SELECT 1 FROM mesh.topic t
+    WHERE t.mesh_id = m.id AND t.name = 'general'
+  );
+
+INSERT INTO mesh.topic_member (topic_id, member_id, role)
+SELECT
+  t.id,
+  mm.id,
+  CASE WHEN m.owner_user_id = mm.user_id THEN 'lead' ELSE 'member' END
+FROM mesh.topic t
+JOIN mesh.mesh m   ON m.id = t.mesh_id
+JOIN mesh.member mm ON mm.mesh_id = t.mesh_id AND mm.revoked_at IS NULL
+WHERE t.name = 'general'
+ON CONFLICT (topic_id, member_id) DO NOTHING;