fix(auth): O(1) selector+verifier token verification

verifyToken loaded every non-revoked token and bcrypt-compared each (O(n) per
request — auth-latency DoS + linear scaling). New token format
vk_<selector>.<verifier>: the non-secret selector is indexed and locates exactly
one row; only the verifier is bcrypt-hashed. Legacy NULL-selector tokens still
verify via a fallback scan. Dropped the useless idx_agent_tokens_hash.

- migration 010_token_selector.sql (adds selector col + unique partial index)
- createToken/verifyToken reworked; also adds listTokenMeta (read for Yerin's
  token_audit tool)
- tests/repos/token_selector.test.js

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/db/repos/agents.js

@@ -42,29 +42,59 @@ export async function setCapabilities(id, capabilities, scopes) {
   return r;
 }
 
+// Token format: vk_<selector>.<verifier>
+//   selector — non-secret, indexed, locates exactly one row (O(1))
+//   verifier — the secret; only this is bcrypt-hashed into token_hash
+// base64url never contains '.', so the delimiter is unambiguous.
 export async function createToken(agent_id, label) {
-  const plaintext = 'vk_' + crypto.randomBytes(32).toString('base64url');
-  const token_hash = await bcrypt.hash(plaintext, 12);
+  const selector = crypto.randomBytes(9).toString('base64url');   // ~12 chars
+  const verifier = crypto.randomBytes(32).toString('base64url');  // ~43 chars
+  const plaintext = `vk_${selector}.${verifier}`;
+  const token_hash = await bcrypt.hash(verifier, 12);
   const { rows: [t] } = await pool.query(
-    `INSERT INTO agent_tokens(agent_id, label, token_hash) VALUES($1,$2,$3) RETURNING id`,
-    [agent_id, label || null, token_hash]
+    `INSERT INTO agent_tokens(agent_id, label, token_hash, selector) VALUES($1,$2,$3,$4) RETURNING id`,
+    [agent_id, label || null, token_hash, selector]
   );
   return { token: plaintext, id: t.id };
 }
 
+const AGENT_SELECT =
+  `SELECT t.id, t.token_hash, t.agent_id, a.*
+   FROM agent_tokens t JOIN agents a ON a.id = t.agent_id`;
+
+async function finalizeVerify(row, secret) {
+  if (await bcrypt.compare(secret, row.token_hash)) {
+    await pool.query(`UPDATE agent_tokens SET last_used=now() WHERE id=$1`, [row.id]);
+    const { token_hash, selector, ...agent } = row;
+    return agent;
+  }
+  return null;
+}
+
 export async function verifyToken(plaintext) {
   if (!plaintext?.startsWith('vk_')) return null;
+  const rest = plaintext.slice(3);
+  const dot = rest.indexOf('.');
+
+  if (dot !== -1) {
+    // New format — single indexed lookup by selector, bcrypt the verifier only.
+    const selector = rest.slice(0, dot);
+    const verifier = rest.slice(dot + 1);
+    const { rows: [row] } = await pool.query(
+      `${AGENT_SELECT} WHERE t.selector = $1 AND t.revoked_at IS NULL`, [selector]
+    );
+    if (!row) return null;
+    return finalizeVerify(row, verifier);
+  }
+
+  // Legacy fallback — pre-migration tokens hashed the full plaintext and have
+  // no selector. Scan only the (shrinking) NULL-selector set.
   const { rows } = await pool.query(
-    `SELECT t.id, t.token_hash, t.agent_id, a.*
-     FROM agent_tokens t JOIN agents a ON a.id = t.agent_id
-     WHERE t.revoked_at IS NULL`
+    `${AGENT_SELECT} WHERE t.selector IS NULL AND t.revoked_at IS NULL`
   );
   for (const row of rows) {
-    if (await bcrypt.compare(plaintext, row.token_hash)) {
-      await pool.query(`UPDATE agent_tokens SET last_used=now() WHERE id=$1`, [row.id]);
-      const { token_hash, ...agent } = row;
-      return agent;
-    }
+    const agent = await finalizeVerify(row, plaintext);
+    if (agent) return agent;
   }
   return null;
 }
@@ -72,3 +102,15 @@ export async function verifyToken(plaintext) {
 export async function revokeToken(token_id) {
   await pool.query(`UPDATE agent_tokens SET revoked_at=now() WHERE id=$1`, [token_id]);
 }
+
+// Token metadata for security review — label/usage/revocation joined with the
+// owning agent. NEVER selects token_hash.
+export async function listTokenMeta() {
+  const { rows } = await pool.query(
+    `SELECT t.id, t.agent_id, a.slug AS agent_slug, a.name AS agent_name,
+            t.label, t.last_used, t.created_at, t.revoked_at
+     FROM agent_tokens t JOIN agents a ON a.id = t.agent_id
+     ORDER BY t.created_at DESC`
+  );
+  return rows;
+}