fix(security): Phase 1 P0 hardening from cross-project audit

Close open uploads, image-proxy SSRF, fail-open AI quotas in production,
auth gaps on app routes, and MCP tenant isolation issues.

Co-authored-by: Cursor <cursoragent@cursor.com>

memento-note/app/api/ai/title-suggestions/route.ts

@@ -3,7 +3,7 @@ import { runLaneWithBillingUser, willUseByokForLane } from '@/lib/ai/provider-fo
 import { getSystemConfig } from '@/lib/config'
 import { auth } from '@/auth'
 import { getAISettings } from '@/app/actions/ai-settings'
-import { reserveUsageOrThrow, QuotaExceededError } from '@/lib/entitlements'
+import { reserveUsageOrThrow, QuotaExceededError, QuotaServiceUnavailableError } from '@/lib/entitlements'
 import { z } from 'zod'
 import { hasUserAiConsent } from '@/lib/consent/server-consent'
 
@@ -68,6 +68,9 @@ export async function POST(req: NextRequest) {
         if (err instanceof QuotaExceededError) {
           return NextResponse.json(err.toJSON(), { status: 402 })
         }
+        if (err instanceof QuotaServiceUnavailableError || process.env.NODE_ENV === 'production') {
+          return NextResponse.json({ error: 'QUOTA_SERVICE_UNAVAILABLE' }, { status: 503 })
+        }
         console.error('[/api/ai/title-suggestions] Quota check error (fail-open):', err)
       }
     }