fix: harden auth, cron secrets, and provider flows (GNU-17) (#148)

- Replace === with crypto.timingSafeEqual in all 7 cron routes via shared lib/auth/cron.ts
- Add in-memory rate limiting (60 req/min) and expires_at support to calendar feed
- Add exponential backoff on MFA verify after 3 failed attempts
- Add 60s cooldown on password reset requests
- Validate bank callback auth code format before API call
- Redact session IDs from bank sync and callback logs
- Validate OAuth redirect_uris against allowlist (claude.ai, claude.com, localhost)
- Remove excessive PII/debug console logging from login page

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/auth/cron.ts

@@ -0,0 +1,32 @@
+import crypto from 'crypto'
+import { NextResponse } from 'next/server'
+
+/**
+ * Verify cron secret using constant-time comparison to prevent timing attacks.
+ * Expects `Authorization: Bearer <CRON_SECRET>` header.
+ *
+ * Returns null if authorized, or a 401 NextResponse if not.
+ */
+export function verifyCronSecret(request: Request): NextResponse | null {
+  const authHeader = request.headers.get('authorization')
+  const cronSecret = process.env.CRON_SECRET
+
+  if (!cronSecret || !authHeader) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+  }
+
+  const token = authHeader.startsWith('Bearer ')
+    ? authHeader.substring(7)
+    : authHeader
+
+  // Use timingSafeEqual to prevent timing-based secret extraction.
+  // Encode both to buffers of equal length by hashing with SHA-256.
+  const tokenHash = crypto.createHash('sha256').update(token).digest()
+  const secretHash = crypto.createHash('sha256').update(cronSecret).digest()
+
+  if (!crypto.timingSafeEqual(tokenHash, secretHash)) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+  }
+
+  return null
+}