feat(broker): invite signature verification + atomic one-time-use

Completes the v0.1.0 security model. Every /join is now gated by a
signed invite that the broker re-verifies against the mesh owner's
ed25519 pubkey, plus an atomic single-use counter.

schema (migrations/0001_demonic_karnak.sql):
- mesh.mesh.owner_pubkey: ed25519 hex of the invite signer
- mesh.invite.token_bytes: canonical signed bytes (for re-verification)
Both nullable; required for new meshes going forward.

canonical invite format (signed bytes):
  `${v}|${mesh_id}|${mesh_slug}|${broker_url}|${expires_at}|
   ${mesh_root_key}|${role}|${owner_pubkey}`

wire format — invite payload in ic://join/<base64url(JSON)> now has:
  owner_pubkey: "<64 hex>"
  signature:    "<128 hex>"

broker joinMesh() (apps/broker/src/broker.ts):
1. verify ed25519 signature over canonical bytes using payload's
   owner_pubkey → else invite_bad_signature
2. load mesh, ensure mesh.owner_pubkey matches payload's owner_pubkey
   → else invite_owner_mismatch (prevents a malicious admin from
   substituting their own owner key)
3. load invite row by token, verify mesh_id matches → else
   invite_mesh_mismatch
4. expiry check → else invite_expired
5. revoked check → else invite_revoked
6. idempotency: if pubkey is already a member, return existing id
   WITHOUT burning an invite use
7. atomic CAS: UPDATE used_count = used_count + 1 WHERE used_count <
   max_uses → if 0 rows affected, return invite_exhausted
8. insert member with role from payload

cli side:
- apps/cli/src/invite/parse.ts: zod-validated owner_pubkey + signature
  fields; client verifies signature immediately and rejects tampered
  links (fail-fast before even touching the broker)
- buildSignedInvite() helper: owners sign invites client-side
- enrollWithBroker sends {invite_token, invite_payload, peer_pubkey,
  display_name} (was: {mesh_id, peer_pubkey, display_name, role})
- parseInviteLink is now async (libsodium ready + verify)

seed-test-mesh.ts generates an owner keypair, sets mesh.owner_pubkey,
builds + signs an invite, stores the invite row, emits ownerPubkey +
ownerSecretKey + inviteToken + inviteLink in the output JSON.

tests — invite-signature.test.ts (9 new):
- valid signed invite → join succeeds
- tampered payload → invite_bad_signature
- signer not the mesh owner → invite_owner_mismatch
- expired invite → invite_expired
- revoked invite → invite_revoked
- exhausted (maxUses=2, 3rd join) → invite_exhausted
- idempotent re-join doesn't burn a use
- atomic single-use: 5 concurrent joins → exactly 1 success, 4 exhausted
- mesh_id payload vs DB row mismatch → invite_mesh_mismatch

verified live: tampered link blocked client-side with a clear error.
Unmodified link joins cleanly end-to-end (roundtrip.ts + join-roundtrip.ts
both pass). 64/64 tests green.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/cli/scripts/make-invite.ts

@@ -1,24 +1,23 @@
 #!/usr/bin/env bun
 /**
- * Build a test invite link from a seeded mesh (reads /tmp/cli-seed.json).
- * Writes the link to stdout.
+ * Emit the signed invite link produced by the broker's seed-test-mesh.
+ *
+ * The seed script (apps/broker/scripts/seed-test-mesh.ts) creates a
+ * mesh with an owner keypair and a signed invite row, then writes
+ * both into /tmp/cli-seed.json. We just echo its inviteLink here so
+ * downstream test scripts can pipe it.
  */
 
 import { readFileSync } from "node:fs";
-import { encodeInviteLink } from "../src/invite/parse";
 
 const seed = JSON.parse(readFileSync("/tmp/cli-seed.json", "utf-8")) as {
-  meshId: string;
+  inviteLink: string;
 };
 
-const link = encodeInviteLink({
-  v: 1,
-  mesh_id: seed.meshId,
-  mesh_slug: "rt-join",
-  broker_url: process.env.BROKER_WS_URL ?? "ws://localhost:7900/ws",
-  expires_at: Math.floor(Date.now() / 1000) + 3600,
-  mesh_root_key: "Y2xhdWRlbWVzaC10ZXN0LW1lc2gta2V5LWRldm9ubHk",
-  role: "member",
-});
-
-console.log(link);
+if (!seed.inviteLink) {
+  console.error(
+    "seed missing inviteLink — re-run apps/broker/scripts/seed-test-mesh.ts",
+  );
+  process.exit(1);
+}
+console.log(seed.inviteLink);