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/suggest-charts/route.ts

@@ -4,7 +4,7 @@ import { willUseByokForLane } from '@/lib/ai/provider-for-user'
 import { getSystemConfig } from '@/lib/config'
 import { prisma } from '@/lib/prisma'
 import { auth } from '@/auth'
-import { reserveUsageOrThrow, QuotaExceededError } from '@/lib/entitlements'
+import { reserveUsageOrThrow, QuotaExceededError, QuotaServiceUnavailableError } from '@/lib/entitlements'
 import { hasUserAiConsent } from '@/lib/consent/server-consent'
 
 export const maxDuration = 30
@@ -63,6 +63,9 @@ export async function POST(req: Request) {
     if (err instanceof QuotaExceededError) {
       return Response.json(err.toJSON(), { status: 402 })
     }
+    if (err instanceof QuotaServiceUnavailableError || process.env.NODE_ENV === 'production') {
+      return Response.json({ error: 'QUOTA_SERVICE_UNAVAILABLE' }, { status: 503 })
+    }
     console.error('[suggest-charts] Quota check error (fail-open):', err)
   }