vanilla-epoll: answer /upload by Content-Length (engine drains large bodies)

The lib now streams (drains) request bodies larger than 1 MiB instead of
buffering them, so for a large upload req.body is empty — but the byte count the
upload profile wants is the declared Content-Length. Answer by
req.content_length() (falls back to the buffered body length when absent, which
also covers small bodies that still take the buffered path).

Depends on enghitalo/vanilla#31 (adds HttpRequest.content_length() + the engine
drain); the Dockerfile clones lib main, so that PR must merge before this builds.
Local (source-built V 0.5.1): upload single-conn 45 req/s/907 MB/s = zix; 32c
303 req/s/6.1 GB/s ~ zix 336/6.7; RSS 14 MB (was ~1 GB buffering). epoll only.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: enghitalo

frameworks/vanilla-epoll/main.v

@@ -145,7 +145,13 @@ fn handle(req_buffer []u8, _fd int, mut out []u8, mut sh Shared) ! {
 		}
 		write_resp(mut out, 'text/plain', sum.str())
 	} else if route == '/upload' {
-		write_resp(mut out, 'text/plain', req.body.len.str())
+		// Answer by Content-Length: the engine streams (drains) large bodies
+		// instead of buffering them, so req.body may be empty — but the declared
+		// length is what the upload profile asks for. Falls back to the buffered
+		// body length when no Content-Length is present (small/bufferable bodies).
+		cl := req.content_length()
+		n := if cl >= 0 { i64(cl) } else { i64(req.body.len) }
+		write_resp(mut out, 'text/plain', n.str())
 	} else if route.starts_with('/json/') {
 		count := clamp_count(parse_u_at(route, 6), sh.dataset.len)
 		mut m := qint(req, qk_m)