feat(icecast): stream access+error logs to stdout/stderr for the Loki tailer (gh-#115)

[genwave-radio]

Closes #115 (P1).

One-file diff: icecast.xml.tmpl sets <accesslog>-</accesslog> + <errorlog>-</errorlog>. Our pinned icecast is 2.5.0, which supports - for BOTH logs (v2.5.0 source: main.c:361 errorlog, main.c:382-383 accesslog → log_open_file(stderr)) — unlike early 2.4.x, so no nginx-style symlink needed. Rotation applies only to the file branch (nothing to neutralize), and restart_logging skips reopen for - logs, so it's HUP-safe (verified live: SIGHUP reload, access lines kept flowing).

🔬 Local evidence (built + ran the image, curled 200 + 404 — from docker logs):

172.17.0.1 - - [29/Jul/2026:12:24:43 +0000] "GET /status.xsl HTTP/1.1" 200 1857 "-" "curl/8.12.1" 0
172.17.0.1 - - [29/Jul/2026:12:24:43 +0000] "GET /definitely-not-a-mount HTTP/1.1" 404 734 "-" "curl/8.12.1" 0
[2026-07-29  12:24:43] WARN fserve/fserve_client_create req for file "…" No such file or directory

Log dir empty before and after HUP — nothing grows on disk. Story221 icecast pins untouched (3/3 pass).

🚀 Ships with the next image release (v2.8.5+). Fleet acceptance after deploy: {station="demo", service="icecast"} shows listener connect/disconnect + mount events in steady state; the alloy service="icecast" label contract is unaffected.