Fix FILE_STORAGE_PORT drift and config docs

[HermanPlay]

Context

File-storage config drift breaks connectivity depending on where the backend runs.

Problems

1. FILE_STORAGE_PORT mismatch — backend .env sets FILE_STORAGE_PORT=8081 (internal server default) but deployment compose publishes internal on 8080 (INTERNAL_PORT=8080). Compose backend/docker-compose.yaml + deployment/docker-compose/docker-compose.yaml override with FILE_STORAGE_PORT=8080, but the committed .env (used for local/non-compose runs) targets 8081 → unreachable. .env.example uses 8888 (public, unsigned) — also wrong.
2. Config comments say API_PORT, code reads APP_PORT — internal/config/config.go (defaultAPIPort, comment block around line 104, line 133) inconsistent; harmless but misleading.
3. Backend no longer needs its own SIGNING_SECRET/SIGNED_URL_SECRET_KEY (SDK delegates /sign to file-storage internal server) — confirm no stale envs remain in compose/docs (see deployment repo, commit 71f6188 already removed some).

Required changes

• Align FILE_STORAGE_PORT everywhere: internal port in use by the file-storage deployment (8080 per current compose INTERNAL_PORT), or make it explicit per environment.
• Fix .env, .env.example; update config comments to match actual var names.
• Update any README/doc references to API_PORT/FILE_STORAGE_PORT.

Acceptance criteria

• Backend can reach file-storage internal API from local (non-compose) run and from compose.
• go test ./... passes.