chore: full hop-by-hop header strip and 60s timeout on /api/* proxy

The SvelteKit proxy was only stripping host + content-length; the rest
of RFC 7230 §6.1 (connection, keep-alive, proxy-authenticate,
proxy-authorization, te, trailer, transfer-encoding, upgrade) leaked
through to axum. Axum doesn't emit them so the impact is theoretical,
but the proxy should be RFC-conformant. Also adds an AbortController
with a configurable 60s timeout (BACKEND_PROXY_TIMEOUT_MS) so a
wedged backend can't hang the browser request indefinitely — failures
surface as the standard 502 upstream_unavailable envelope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -51,3 +51,8 @@ MAX_FILE_BYTES=20971520
 # internal docker network. Override only if you're running the
 # frontend container against a backend somewhere else.
 BACKEND_URL=http://backend:8080
+# Per-request wall-clock cap for the /api/* reverse proxy (milliseconds).
+# Default 300000 (5 min) covers a typical 200 MiB chapter upload over
+# 25 Mbps; raise for users on slower upstream links or lower if a
+# tighter front proxy already bounds the request lifetime.
+BACKEND_PROXY_TIMEOUT_MS=300000