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>

app/api/extensions/enable-banking/callback/route.ts

@@ -69,6 +69,12 @@ export async function GET(request: Request) {
     return NextResponse.redirect(`${baseUrl}/settings?bank_error=missing_parameters`)
   }
 
+  // Validate authorization code format
+  const codePattern = /^[a-zA-Z0-9._~+\/-]{8,2048}$/
+  if (!codePattern.test(code)) {
+    return NextResponse.redirect(`${baseUrl}/settings?bank_error=invalid_code_format`)
+  }
+
   const supabase = await createServiceClient()
 
   try {
@@ -105,7 +111,7 @@ export async function GET(request: Request) {
 
     console.log('[enable-banking] Session created successfully', {
       connectionId: pendingConnection.id,
-      sessionId: session_id,
+      sessionId: '[REDACTED]',
       accountCount: accounts.length,
       consentExpiresAt,
     })
@@ -150,7 +156,7 @@ export async function GET(request: Request) {
       console.error('[enable-banking] Failed to update connection after session creation', {
         connectionId: pendingConnection.id,
         updateError: { message: updateError.message, code: updateError.code, details: updateError.details },
-        sessionId: session_id,
+        sessionId: '[REDACTED]',
       })
       throw new Error(`Failed to update connection: ${updateError.message}`)
     }