feat(downloads): a per-site failure catalog — Facebook's walls say why and what to do

A failed Facebook download read "Download failed": yt-dlp words the login wall
(18+ reel, closed group) as "Cannot parse data", which no category matched.
Measured on dev with the owner's cookie before writing a rule — the same text is
the login wall without a cookie and a refused cookie with one.

- errors.diagnose(msg, url, cookie_attached) tries a host-scoped catalog first
  (all-of markers, cookie state, link fragment) and falls back to the category.
  First entries: facebook.login_wall, facebook.cookie_rejected,
  facebook.group_post_video. The module doc is the recipe for the next entry.
- download_jobs.error_detail (0084) + backfill of the stored Facebook parse
  failures that were unknown.
- The pipeline's failure branch passes the link and whether the cookie went
  with it; resume and redownload clear code and detail (redownload never
  cleared error_code).
- A job meeting an already-failed asset got the text but no code, so it read
  "Download failed" where the first job said why. It is diagnosed again, with
  the cookie state from credentials.is_authed_asset (now shared with the
  cookie gate); the redacted network text classifies back to network.
- FE: failCopyKeys gives the row and the failure window one lookup — catalog
  copy, else the category. Copy in EN and HU; a contract test pins every
  backend entry to both locales.

backend/tests/test_download_errors.py

@@ -64,6 +64,71 @@ def test_our_own_worker_refusals_classify_as_themselves():
     assert classify(dlerrors.QUOTA_FULL_MESSAGE) == "quota_exceeded"
 
 
+def test_the_redacted_network_text_classifies_back_to_network():
+    # The redacted text is also what a failed asset keeps, and a second job that meets that asset is
+    # diagnosed from it — without its own rule it would read "Download failed".
+    assert classify(dlerrors.NETWORK_REDACTED_MESSAGE) == "network"
+
+
+# Verbatim yt-dlp 2026.07.04 output from prod jobs, measured on dev 2026-09-13 (E11.5).
+_FB_PARSE = (
+    "ERROR: [facebook] 1736360027626346: Cannot parse data; please report this issue on  "
+    "https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template."
+)
+_FB_NO_FORMATS = (
+    "ERROR: [facebook] 1755988225525547: No video formats found!; please report this issue on  "
+    "https://github.com/yt-dlp/yt-dlp/issues?q= , filling out the appropriate issue template."
+)
+_REEL = "https://www.facebook.com/reel/1736360027626346/?fs=e&fs=e"
+_GROUP_POST = "https://www.facebook.com/groups/983554479435596/permalink/1755988225525547/?"
+
+
+def test_facebook_cannot_parse_data_is_the_login_wall_without_a_cookie():
+    for url in (
+        _REEL,
+        _GROUP_POST,
+        "https://fb.watch/Ia_iJYI2Vw/?fs=e",
+        "https://m.facebook.com/x",
+    ):
+        assert dlerrors.diagnose(_FB_PARSE, url=url) == ("login_required", "facebook.login_wall")
+
+
+def test_facebook_cannot_parse_data_with_a_cookie_is_the_cookie_refused():
+    # Telling a user who already gave a cookie to add one is the failure this entry exists to avoid.
+    assert dlerrors.diagnose(_FB_PARSE, url=_REEL, cookie_attached=True) == (
+        "login_required",
+        "facebook.cookie_rejected",
+    )
+
+
+def test_a_facebook_group_post_without_formats_names_the_link():
+    assert dlerrors.diagnose(_FB_NO_FORMATS, url=_GROUP_POST, cookie_attached=True) == (
+        "format_unavailable",
+        "facebook.group_post_video",
+    )
+    # A reel with no formats is another failure: only the category.
+    assert dlerrors.diagnose(_FB_NO_FORMATS, url=_REEL, cookie_attached=True) == (
+        "format_unavailable",
+        None,
+    )
+
+
+def test_a_catalog_entry_never_leaves_its_host():
+    # "Cannot parse data" is yt-dlp's wording for a broken extractor on every site.
+    other = _FB_PARSE.replace("[facebook]", "[vimeo]")
+    assert dlerrors.diagnose(other, url="https://vimeo.com/1") == ("unknown", None)
+    # The Facebook text for a link that is not Facebook's (a look-alike host) is not the Facebook wall.
+    assert dlerrors.diagnose(_FB_PARSE, url="https://facebook.com.evil/reel/1") == ("unknown", None)
+    # No link known: the category only.
+    assert dlerrors.diagnose(_FB_PARSE) == ("unknown", None)
+
+
+def test_diagnose_falls_back_to_the_category():
+    msg = "ERROR: [facebook] 123: You must be logged in to see this post"
+    assert dlerrors.diagnose(msg, url=_REEL) == ("login_required", None)
+    assert dlerrors.diagnose(None, url=_REEL) == ("unknown", None)
+
+
 def test_a_url_jobs_network_failure_is_redacted_but_youtube_keeps_its_text():
     # SEC-12: the raw transport text for a user-chosen host:port is a port-scanner readout, so it is
     # replaced for `url` jobs. Everything else must survive verbatim, or a real diagnosis is lost.