task/29: fix vouch idempotency + replay guard — unique constraints + DB push

## Code review round 2 issues resolved

### Vouch replay / duplicate boost vulnerability — FIXED
- `nostr-trust-vouches.ts` schema: added `eventId` column + two unique guards:
  1. `UNIQUE(event_id)` — same signed event cannot be replayed for any pair
  2. `UNIQUE INDEX uq_nostr_trust_vouches_pair(voucher_pubkey, vouchee_pubkey)` —
     each elite may vouch for a given target exactly once
- Route: insert now uses `.onConflictDoNothing().returning({ id })`
  - If returned array is empty → duplicate detected → 409 with existing state,
    no trust boost applied
  - If returned array has rows → first-time vouch → boost applied exactly once
- `eventId` extracted from `ev["id"]` (NIP-01 sha256 event id) before insert
- Migration file `0006_timmy_economic_peer.sql` updated to include both
  unique constraints (UNIQUE + CREATE UNIQUE INDEX)
- Schema pushed to production — all three indexes confirmed in DB:
  `nostr_trust_vouches_event_id_unique`, `uq_nostr_trust_vouches_pair`, `pkey`

### Previously fixed (round 1)
- LNURL-pay resolution in ZapService (full NIP-57 §4 flow)
- Vouch event made required with p-tag vouchee binding
- DB migration file 0006 created for both new tables + lightning_address column
- GET /identity/timmy now returns relayUrl field

### Verified
- TypeScript: 0 errors (tsc --noEmit clean)
- DB: all constraints confirmed live in production
- API: /identity/timmy 200, /identity/challenge nonce, /identity/vouch 401/400

lib/db/migrations/0006_timmy_economic_peer.sql

@@ -22,18 +22,26 @@ CREATE INDEX IF NOT EXISTS idx_timmy_nostr_events_recipient
   ON timmy_nostr_events(recipient_pubkey);
 
 -- ── nostr_trust_vouches ───────────────────────────────────────────────────────
--- Records elite-tier co-signing events. Each row grants a trust boost to the
--- vouchee and stores the raw Nostr event for auditability.
+-- Records elite-tier co-signing events. Each row grants a one-time trust boost
+-- to the vouchee. Two uniqueness guards prevent trust fraud:
+--   1. (voucher_pubkey, vouchee_pubkey) — one vouch per pair, ever.
+--   2. event_id — a specific signed event cannot be replayed.
 
 CREATE TABLE IF NOT EXISTS nostr_trust_vouches (
   id                TEXT PRIMARY KEY,
   voucher_pubkey    TEXT NOT NULL REFERENCES nostr_identities(pubkey),
   vouchee_pubkey    TEXT NOT NULL,
+  event_id          TEXT NOT NULL,
   vouch_event_json  TEXT NOT NULL,
   trust_boost       INTEGER NOT NULL DEFAULT 20,
-  created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW()
+  created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+
+  CONSTRAINT uq_nostr_trust_vouches_event_id UNIQUE (event_id)
 );
 
+CREATE UNIQUE INDEX IF NOT EXISTS uq_nostr_trust_vouches_pair
+  ON nostr_trust_vouches(voucher_pubkey, vouchee_pubkey);
+
 CREATE INDEX IF NOT EXISTS idx_nostr_trust_vouches_voucher
   ON nostr_trust_vouches(voucher_pubkey);