test: add timing attack tests for bot-auth and use timingSafeEqual

Replace manual XOR loop with Node's native crypto.timingSafeEqual for
constant-time secret comparison. Add tests verifying identical error
messages for wrong secrets and <1ms response time variance across 100
requests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/middleware/bot-auth.ts

@@ -2,7 +2,7 @@
 // Verifies bot identity via bot_id + secret (SHA256 hash comparison).
 // Supports: Authorization header or query params.
 
-import { createHash } from 'crypto'
+import { createHash, timingSafeEqual } from 'crypto'
 import { db, schema } from '../db/index.js'
 import { eq } from 'drizzle-orm'
 import type { Context } from 'hono'
@@ -55,16 +55,10 @@ export async function authenticateBot(c: Context): Promise<BotAuthContext | Resp
     return c.json({ error: 'Invalid bot_id or secret.' }, 401)
   }
 
-  // Constant-time comparison
+  // Constant-time comparison using Node's native timingSafeEqual
   const expected = rows[0].secretHash
-  if (hash.length !== expected.length) {
-    return c.json({ error: 'Invalid bot_id or secret.' }, 401)
-  }
-  let mismatch = 0
-  for (let i = 0; i < hash.length; i++) {
-    mismatch |= hash.charCodeAt(i) ^ expected.charCodeAt(i)
-  }
-  if (mismatch !== 0) {
+  if (hash.length !== expected.length ||
+      !timingSafeEqual(Buffer.from(hash), Buffer.from(expected))) {
     return c.json({ error: 'Invalid bot_id or secret.' }, 401)
   }