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/sysconfig.py

@@ -140,7 +140,7 @@ SPECS: tuple[ConfigSpec, ...] = (
         max=100_000,
     ),
     # min=1, NOT 0: a stored 0 used to read as "no per-file cap" (the cap was enforced as
-    # `if cap and size > cap`, so a falsy value disabled it). Since P1 the route passes this number
+    # `if cap and size > cap`, so a falsy value disabled it). Now the route passes this number
     # straight to `capped_stream_to`, where 0 would refuse every upload instead — the opposite
     # failure, and the reason `config.download_upload_max_bytes` carries its own `ge=1`. Either way
     # a stored 0 is wrong, so it is floored at 1; the overall footprint is separately bounded by the