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/app/plex/client.py

@@ -2,7 +2,7 @@
 
 Server-side ONLY: uses the admin ``X-Plex-Token`` from sysconfig. Playback never goes through
 this client — Siftlode plays the LOCAL physical file (see app.plex.paths). Plex is used purely for
-metadata, image proxying, and (P5) optional watch-state write-back. Requests JSON via the
+metadata, image proxying, and optional watch-state write-back. Requests JSON via the
 ``Accept: application/json`` header (Plex serves XML by default).
 """
 
@@ -65,7 +65,7 @@ class PlexClient:
         except ValueError as e:
             raise PlexError("Plex returned a non-JSON response") from e
 
-    # --- Connectivity + sections (P0) ---
+    # --- Connectivity + sections ---
     def server_info(self) -> dict:
         """Root identity (friendlyName / version) — used by the admin 'Test connection'."""
         return self._get("/")
@@ -75,7 +75,7 @@ class PlexClient:
         mc = self._get("/library/sections")
         return mc.get("Directory", []) or []
 
-    # --- Catalog mirror (P1) ---
+    # --- Catalog mirror ---
     def section_items(
         self, key: str, item_type: int, start: int = 0, size: int = 200
     ) -> tuple[list[dict], int | None]:
@@ -137,7 +137,7 @@ class PlexClient:
         mc = self._get(f"/library/collections/{rating_key}/children")
         return mc.get("Metadata", []) or []
 
-    # --- Collection editing (P2 — write-back to Plex; all shared/global, admin-gated in the routes) ---
+    # --- Collection editing (write-back to Plex; all shared/global, admin-gated in the routes) ---
     @property
     def machine_id(self) -> str:
         """Server machineIdentifier — needed in the `uri` of collection add/create calls. Cached.
@@ -206,7 +206,7 @@ class PlexClient:
         items = mc.get("Metadata", []) or []
         return items[0] if items else None
 
-    # --- Watch-state write-back (P5 Phase B — Siftlode→Plex push; acts on the token's account) ---
+    # --- Watch-state write-back (Siftlode→Plex push; acts on the token's account) ---
     _LIBRARY_IDENTIFIER = "com.plexapp.plugins.library"
 
     def scrobble(self, rating_key: str) -> None:
@@ -243,7 +243,7 @@ class PlexClient:
             f"&state={state}&time={int(time_ms)}&duration={int(duration_ms)}",
         )
 
-    # --- Watch-state read for incremental sync (P5 Phase C) ---
+    # --- Watch-state read for incremental sync ---
     def accounts(self) -> list[dict]:
         """Server accounts (the owner + any managed/home users): id, name. Used to resolve the
         owner's accountID so the shared watch-history feed can be filtered to just their views."""