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/app/routes/downloads.py

@@ -1033,7 +1033,7 @@ def list_links(
 
 class CreateLinkIn(BaseModel):
     password: str = ""
-    # D1: a newly shared download is downloadable by default — the common intent is "let the
+    # A newly shared download is downloadable by default — the common intent is "let the
     # recipient have the file", not stream-only. Forward-only: this only sets the default for links
     # minted from now on; existing links keep whatever `allow_download` they were created with (the
     # column default stays False, so no back-catalogue link silently flips). The owner can still