fix(gate): the plan-code guard catches bare sprint and phase codes, and the purge takes the P ones

The sprint alternative matched only `(S6)` or `S3e2`, so the commonest purged form (`# S6:`,
`S3's`, `"""S6 —`, `S1 justified`) and every phase code (`P1`) would come back unflagged; the phase
codes had in fact survived the purge in 16 lines (Plex client sections, the upload's header shape),
now reworded around them.

One pattern now covers epic, roadmap, sprint and phase codes; the named exceptions are TV episode
notation, a fixture's season or episode title (`title="S1"`) and ruff's `E4/E7/E9`, and a ruff rule
like `S110` does not match. The case test lists the purged forms and no longer claims every form.
audit_metrics copies the definition (ASCII, as JavaScript's \b is) and reports it as
plan_code_lines / plan_code_files.

backend/tests/conftest.py

@@ -254,7 +254,7 @@ def as_user(db, override_dep, _configured):
 
 @pytest.fixture
 def upload_headers():
-    """Build the headers `POST /api/downloads/upload` takes since P1.
+    """Build the headers `POST /api/downloads/upload` takes (the file is the raw body).
 
     Shared, because two test modules drive that route and a second copy of this is exactly the drift
     that lets the wire shape change in one place only. The body IS the file now; the names ride as