feat: add server-side quota tracking with DynamoDB (#379)

- Add dynamo-quota-manager.ts for atomic quota checks using ConditionExpression
- Enforce daily request limit, daily token limit, and TPM limit
- Return 429 with quota details (type, used, limit) when exceeded
- Quota is opt-in: only enabled when DYNAMODB_QUOTA_TABLE env var is set
- Remove client-side quota enforcement (server is now source of truth)
- Simplify use-quota-manager.tsx to only display toasts
- Add @aws-sdk/client-dynamodb dependency

package.json

@@ -32,6 +32,7 @@
         "@ai-sdk/google": "^3.0.0",
         "@ai-sdk/openai": "^3.0.0",
         "@ai-sdk/react": "^3.0.1",
+        "@aws-sdk/client-dynamodb": "^3.957.0",
         "@aws-sdk/credential-providers": "^3.943.0",
         "@formatjs/intl-localematcher": "^0.7.2",
         "@langfuse/client": "^4.4.9",