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/chat/route.ts

@@ -9,7 +9,7 @@ import { auth } from '@/auth'
 import { hasUserAiConsent } from '@/lib/consent/server-consent'
 import { loadTranslations, getTranslationValue, SupportedLanguage } from '@/lib/i18n'
 import { toolRegistry } from '@/lib/ai/tools'
-import { reserveUsageOrThrow, QuotaExceededError } from '@/lib/entitlements'
+import { reserveUsageOrThrow, QuotaExceededError, QuotaServiceUnavailableError } from '@/lib/entitlements'
 import { ByokUnavailableError } from '@/lib/byok'
 import { trackFeatureUsage } from '@/lib/usage-tracker'
 import { readFile } from 'fs/promises'
@@ -78,7 +78,15 @@ export async function POST(req: Request) {
         { status: 503 }
       )
     }
-    console.error('[chat] Quota check error (fail-open):', err)
+    if (err instanceof QuotaServiceUnavailableError) {
+      return Response.json({ error: err.code }, { status: 503 })
+    }
+    if (process.env.NODE_ENV !== 'production') {
+      console.error('[chat] Quota check error (fail-open):', err)
+    } else {
+      console.error('[chat] Quota check error:', err)
+      return Response.json({ error: 'QUOTA_SERVICE_UNAVAILABLE' }, { status: 503 })
+    }
   }
 
   // 2. Parse request body