relay_frames_received_total was only incremented in broadcast(), missing
events dropped by inactive-account filtering or validation failure.
move the counter to subscriber's serverMessage after frame decode, matching
the Go relay's events_received_counter which counts at processRepoEvent entry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

the 64KB stack buffer overflows with 2800+ hosts, causing 502 responses.
switch to ArrayList writer for unbounded JSON response. also adds the
missing checkAdmin gate that other admin endpoints already have.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Replace hand-rolled HTTP parser with zig 0.15's std.http.Server,
which provides buffered I/O that correctly handles POST bodies
split across TCP segments behind traefik. This is the proper fix
for the issue that commit 4733560 worked around with a multi-read loop.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>