feat: invalidate metadata cache after avatar and header uploads

[bigint]

Summary

• Add metadata webhook support for avatar and header uploads on supported networks.
• Queue cache invalidation asynchronously with waitUntil so upload responses stay fast.
• Introduce webhook signing, environment typings, and config for the new metadata endpoint.
• Cover the webhook helper and route integration with unit and integration tests.

Testing

• Added unit tests for webhook payloads, signatures, supported networks, missing config, and failure handling.
• Added integration tests for successful uploads, unsupported networks, webhook errors, and upload failures.
• Not run (not requested)