Add missing nginx directives recommended by Immich docs for mobile
photo backup over slow links:
- http2 on: multiplex parallel asset uploads from the mobile app
- client_body_timeout / send_timeout / keepalive_timeout 600s: cover
the client<->nginx leg (default 60s matched the Android timeout)
- proxy_request_buffering off + proxy_buffering off: stream upload
bytes to immich as they arrive instead of buffering the whole file,
keeping the TCP connection active and avoiding idle-socket timeouts
- proxy_connect_timeout 600s: explicit upstream connect timeout