security(p2p): close escrow-RPC anon-drain (release/lock behind signed edge functions)

Round 2 — fixes the CRITICAL discovered during round 1 (as severe as the withdrawal
BOLA): lock_/release_/refund_escrow_internal had default PUBLIC EXECUTE and the client
called release_escrow_internal directly with the anon key, so anyone could drain any
victim's LOCKED balance.

- release_escrow_internal now reachable only via new signed confirm-payment edge
  function: verifies seller wallet signature (raw + <Bytes>), asserts the wallet owns
  the seller identity, derived user_id == trade.seller_id, trade == payment_sent,
  single-use nonce, then release runs with the service role. confirmPaymentReceived
  now invokes confirm-payment (no more anon rpc).
- lock_escrow_internal now behind new signed lock-escrow edge function (a caller can
  only lock its own balance; stops griefing a victim's balance). Removed a zero-amount
  no-op lock call.
- Migration 20260725030000: REVOKE EXECUTE on lock_/release_/refund_escrow_internal
  from PUBLIC/anon/authenticated; GRANT to service_role only. refund has no direct
  client caller (service-role + admin_resolve_dispute only).
- DEPLOY_RUNBOOK.md consolidates the ordered migrations, edge functions and secrets
  for both security rounds. Migration 20260725030000 must ship WITH the two new edge
  functions + frontend or offer-create/payment-release break.

Remaining (non-fund, Round 2+ follow-up): read RPCs still key on a non-secret user_id
(binding to a signed session would force a sign-prompt on every passive balance read —
deferred); p2p_fiat_offers/trades/messages retain USING(true) (status labels move no
funds now that all escrow movement is service-role-gated).

web/supabase/functions/_shared/identity-auth.ts

@@ -141,6 +141,50 @@ export function buildDepositChallenge(p: {
   ].join('\n')
 }
 
+/**
+ * Canonical challenge for a seller confirming payment / releasing escrow.
+ * MUST be byte-identical to the client (shared/lib/p2p-fiat.ts).
+ */
+export function buildConfirmPaymentChallenge(p: {
+  tradeId: string
+  sellerIdentity: string
+  signerAddress: string
+  timestamp: number
+  nonce: string
+}): string {
+  return [
+    'Pezkuwi P2P Confirm Payment',
+    `trade:${p.tradeId}`,
+    `identity:${p.sellerIdentity}`,
+    `signer:${p.signerAddress}`,
+    `timestamp:${p.timestamp}`,
+    `nonce:${p.nonce}`,
+  ].join('\n')
+}
+
+/**
+ * Canonical challenge for locking one's OWN balance into escrow (offer create).
+ * MUST be byte-identical to the client (shared/lib/p2p-fiat.ts).
+ */
+export function buildLockEscrowChallenge(p: {
+  identityId: string
+  token: string
+  amount: number
+  signerAddress: string
+  timestamp: number
+  nonce: string
+}): string {
+  return [
+    'Pezkuwi P2P Lock Escrow',
+    `identity:${p.identityId}`,
+    `token:${p.token}`,
+    `amount:${p.amount}`,
+    `signer:${p.signerAddress}`,
+    `timestamp:${p.timestamp}`,
+    `nonce:${p.nonce}`,
+  ].join('\n')
+}
+
 /**
  * Canonical challenge string for a privileged admin action (dispute claim /
  * resolve). MUST be byte-identical to the client (DisputeResolutionPanel).