feat(auth): NIP-98 nostr sign-in for Smirk; fix SHA256 fallback + /status BOLA

Adds schnorr (BIP-340) NIP-98 verification (kind 27235) as the preferred Smirk login, with the legacy Ed25519 wallet-message path kept as a feature-detected fallback for wallets without signNostrEvent. New src/utils/nip98.js verifies securely: rebuilds a clean event, independently recomputes getEventHash and compares to event.id (defeats nostr-tools' verify-memo footgun), then schnorr-verifies; enforces kind, created_at freshness, and exactly-one u/method/challenge tags bound to the server nonce.

auth.js: /verify branches on body.event (NIP-98) vs {publicKey,signature,challenge} (legacy). Challenge consume is now atomic single-use (UPDATE ... WHERE used=FALSE AND expires_at>NOW() RETURNING) before verification. Removed the deprecated SHA256(challenge) pre-hash fallback (past its removal date). /status is now session-gated (X-Session-Token vs users.anon_token; 401/403 on mismatch) — closes the unauthenticated per-socketId disclosure. Client smirkAuth.js: feature-detects signNostrEvent, signs the NIP-98 event, falls back to legacy; checkStatus() now sends the session token.

Adds nostr-tools ^2.23.9 (works under CJS in prod; test loader falls back to its self-contained bundle under Jest). New adversarial suite test/smirkNip98_ultra.test.js (13 tests: tamper/expiry/wrong-tag/bad-sig/memo-bypass/replay). Suite 47/330 green.

src/package.json

@@ -33,6 +33,7 @@
     "dotenv": "^16.0.3",
     "express": "^4.21.3",
     "node-pg-migrate": "^8.0.4",
+    "nostr-tools": "^2.23.9",
     "pg": "^8.15.5",
     "qrcode": "^1.5.3",
     "resend": "^4.0.0",