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/src/crypto.ts

@@ -28,6 +28,47 @@ export function canonicalHello(
   return `${meshId}|${memberId}|${pubkey}|${timestamp}`;
 }
 
+/** Canonical invite bytes — everything in the payload except the signature. */
+export function canonicalInvite(fields: {
+  v: number;
+  mesh_id: string;
+  mesh_slug: string;
+  broker_url: string;
+  expires_at: number;
+  mesh_root_key: string;
+  role: "admin" | "member";
+  owner_pubkey: string;
+}): string {
+  return `${fields.v}|${fields.mesh_id}|${fields.mesh_slug}|${fields.broker_url}|${fields.expires_at}|${fields.mesh_root_key}|${fields.role}|${fields.owner_pubkey}`;
+}
+
+/**
+ * Verify an ed25519 signature over arbitrary canonical bytes.
+ * Used by invite verification + (future) any other signed payload.
+ */
+export async function verifyEd25519(
+  canonicalText: string,
+  signatureHex: string,
+  pubkeyHex: string,
+): Promise<boolean> {
+  if (
+    !/^[0-9a-f]{64}$/i.test(pubkeyHex) ||
+    !/^[0-9a-f]{128}$/i.test(signatureHex)
+  ) {
+    return false;
+  }
+  const s = await ensureSodium();
+  try {
+    return s.crypto_sign_verify_detached(
+      s.from_hex(signatureHex),
+      s.from_string(canonicalText),
+      s.from_hex(pubkeyHex),
+    );
+  } catch {
+    return false;
+  }
+}
+
 export const HELLO_SKEW_MS = 60_000;
 
 /**