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/broker/scripts/seed-test-mesh.ts

@@ -12,18 +12,23 @@
 import { eq } from "drizzle-orm";
 import sodium from "libsodium-wrappers";
 import { db } from "../src/db";
-import { mesh, meshMember } from "@turbostarter/db/schema/mesh";
+import { invite, mesh, meshMember } from "@turbostarter/db/schema/mesh";
 import { user } from "@turbostarter/db/schema/auth";
+import { canonicalInvite } from "../src/crypto";
 
 const USER_ID = "test-user-smoke";
 const MESH_SLUG = "smoke-test";
+const BROKER_URL = process.env.BROKER_WS_URL ?? "ws://localhost:7900/ws";
 
 async function main() {
   // Generate real ed25519 keypairs so crypto_box (via ed25519→X25519
   // conversion) works in Step 18+ round-trip tests.
   await sodium.ready;
+  const kpOwner = sodium.crypto_sign_keypair();
   const kpA = sodium.crypto_sign_keypair();
   const kpB = sodium.crypto_sign_keypair();
+  const OWNER_PUBKEY = sodium.to_hex(kpOwner.publicKey);
+  const OWNER_SECRET = sodium.to_hex(kpOwner.privateKey);
   const PEER_A_PUBKEY = sodium.to_hex(kpA.publicKey);
   const PEER_A_SECRET = sodium.to_hex(kpA.privateKey);
   const PEER_B_PUBKEY = sodium.to_hex(kpB.publicKey);
@@ -53,6 +58,7 @@ async function main() {
       name: "Smoke Test",
       slug: MESH_SLUG,
       ownerUserId: USER_ID,
+      ownerPubkey: OWNER_PUBKEY,
       visibility: "private",
       transport: "managed",
       tier: "free",
@@ -60,6 +66,40 @@ async function main() {
     .returning({ id: mesh.id });
   if (!m) throw new Error("mesh insert failed");
 
+  // Build + sign an invite, store it so /join can verify.
+  const expiresAtSec = Math.floor(Date.now() / 1000) + 3600;
+  const invitePayload = {
+    v: 1 as const,
+    mesh_id: m.id,
+    mesh_slug: MESH_SLUG,
+    broker_url: BROKER_URL,
+    expires_at: expiresAtSec,
+    mesh_root_key: "c21va2UtdGVzdC1tZXNoLXJvb3Qta2V5LWRldg",
+    role: "member" as const,
+    owner_pubkey: OWNER_PUBKEY,
+  };
+  const canonical = canonicalInvite(invitePayload);
+  const signature = sodium.to_hex(
+    sodium.crypto_sign_detached(
+      sodium.from_string(canonical),
+      kpOwner.privateKey,
+    ),
+  );
+  const fullPayload = { ...invitePayload, signature };
+  const token = Buffer.from(JSON.stringify(fullPayload), "utf-8").toString(
+    "base64url",
+  );
+  await db.insert(invite).values({
+    meshId: m.id,
+    token,
+    tokenBytes: canonical,
+    maxUses: 5,
+    usedCount: 0,
+    role: "member",
+    expiresAt: new Date(expiresAtSec * 1000),
+    createdBy: USER_ID,
+  });
+
   const [peerA] = await db
     .insert(meshMember)
     .values({
@@ -84,6 +124,10 @@ async function main() {
 
   const seed = {
     meshId: m.id,
+    ownerPubkey: OWNER_PUBKEY,
+    ownerSecretKey: OWNER_SECRET,
+    inviteToken: token,
+    inviteLink: `ic://join/${token}`,
     peerA: {
       memberId: peerA.id,
       pubkey: PEER_A_PUBKEY,