docs(cctv): state that offline Catalonia cameras show the publisher's still

The Catalonia feed has no camera status field, so an offline camera keeps
its catalog entry and SCT or Barcelona answer its frame with their own
"image unavailable" still (HTTP 200). Say so in DATA_SOURCES.md and stop
the loader comment from implying every camera returns a live frame.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

server/providers/cctv/sources.js

@@ -1733,9 +1733,12 @@ export function cataloniaCameraName(carretera, municipi) {
  * Fetch Catalonia traffic cameras (Servei Català de Trànsit), keyless: one
  * WFS/GML XML list. The feed carries a `font` field naming the real
  * publisher per camera; only SCT's own highway cameras and the hotlinked
- * Barcelona (IMI) and Terrassa municipal cameras are kept, each confirmed to
- * return a real frame — frame URLs are checked against a host allowlist
- * (CATALONIA_IMAGE_HOSTS), so a row from any other publisher is dropped.
+ * Barcelona (IMI) and Terrassa municipal cameras are kept — the publishers
+ * whose frame links were confirmed to serve images. Frame URLs are checked
+ * against a host allowlist (CATALONIA_IMAGE_HOSTS), so a row from any other
+ * publisher is dropped. The feed carries no camera status, so a camera its
+ * publisher has taken offline still registers and serves that publisher's
+ * own "image unavailable" still.
  * Each non-SCT camera carries its publisher as a `credit` alongside the
  * shared `provider`. The feed carries no heading, so every camera takes the
  * id-hash fallback, low-confidence pose (same as TfL and Fintraffic).