sofatutor/llm-proxy

AI Summary: The goal of this issue is to increase the Go test coverage for internal packages (`./internal/...`) to at least 90%. This work is to be done on a new branch created from `feature/proxy-caching`, focusing on adding tests for critical components like `internal/proxy` (caching, TTL, auth, streaming, Redis fallbacks), `internal/middleware` (cache-hit bypass), and `internal/server` (env wiring), including uncovered error paths.