fix(downloads): a site the downloader no longer understands says so, and offers a retry

yt-dlp asks for a bug report on an extraction failure it did not expect: the site answered in a way its extractor does not read (measured on a TikTok link: fails on every attempt under yt-dlp 2026.07.04, downloads under 2026.08.19). That failure read as "Something went wrong" with no retry; it is now its own category, site_changed, with a Retry in the failure window. A contract test holds every failure category to its copy in both languages.

backend/app/downloads/errors.py

@@ -144,6 +144,13 @@ _RULES: tuple[tuple[str, str], ...] = (
     ("postprocessing", "postprocess"),
     ("produced no output file", "postprocess"),
     ("no output file", "postprocess"),
+    # yt-dlp's own verdict that its extractor did not understand the site's answer: it appends this
+    # request for a bug report to an UNEXPECTED extraction failure, never to an expected one (a private,
+    # removed or geo-blocked video says so in words matched above). Measured 2026-10-12: a TikTok link
+    # failed with it on every attempt under yt-dlp 2026.07.04 and downloads under 2026.08.19 — the
+    # site changed and a newer yt-dlp is the cure. After the media-format and post-processing rules,
+    # whose texts can carry the same request; before the network ones, which are about the transport.
+    ("please report this issue", "site_changed"),
     # Network / transport.
     ("unable to download", "network"),
     ("http error", "network"),