fix(gate): the plan-code guard catches decision codes, and the purge takes them

A plan's decision ids (`D1`, `D4`, `D6-A`) cited in 11 comments and test titles named a document
no reader of this repo has, and the guard had no pattern for them. It has now (mirrored in
audit_metrics), with cases for each form the citations took and for the look-alikes it must leave
alone (a JPEG marker, "3D"); the 11 lines say their why without the id. Case test seen red without
the pattern, the scan red on a planted `D9:`.

backend/tests/test_links.py

@@ -99,7 +99,7 @@ class TestPublicMetaAudioFlag:
 
 
 class TestCreateLinkDefault:
-    """D1: a newly minted share link is downloadable by default. The one-click Share relies on
+    """A newly minted share link is downloadable by default. The one-click Share relies on
     this — it POSTs no `allow_download`, so the API default decides. Forward-only lives here (the API
     default), NOT in a migration: the DownloadLink column default stays False, so no existing back-
     catalogue link silently flips to downloadable."""