Add rate limiting to solution submission and voting endpoints

[Mkalbani]

Description

With the queue system, comments, and voting/ranking endpoints now live, the API has no protection against spam submissions or vote manipulation. Add endpoint-level rate limiting so a single user or IP can't flood the queue or skew rankings through rapid repeated requests.

Acceptance Criteria

• @nestjs/throttler (or equivalent) integrated at the module level
• Stricter limits applied to POST /solutions, POST /solutions/:id/vote, and POST /comments than to read endpoints
• Rate-limit config values (window, max requests) are environment-configurable, not hardcoded
• Exceeding the limit returns a 429 with a Retry-After header, not a generic error
• Existing auth service (from the websocket JWT work) is used to key limits per-user where a valid JWT is present, falling back to IP for anonymous requests
• Integration tests cover both throttled and non-throttled request paths