fix: Implement rate limiting for API server

[echobt]

This PR addresses the issue of missing rate limiting in the API server.

Changes

• Added tower-governor dependency for rate limiting.
• Configured rate limiting middleware in run_server with 60 requests per second and a burst of 10.
• This prevents simple DoS attacks and resource exhaustion from abuse of the embedding generation endpoint.

Verification

• Verified that the rate limiting middleware is correctly applied.
• Ran existing tests to ensure no regressions.