nginx does not automatically handle URI appending when variables are used
in proxy_pass. Replace variable-based proxy_pass with hardcoded IP + rewrite
rules to correctly map /api/v2/ts/* → /api/v1/ts/* on the Rust htyts backend.
Symptom: all requests to /api/v2/ts/ returned 404 because the variable-based
proxy_pass sent every request to /api/v1/ts/ (root) regardless of path.