chore(comments): drop epic, sprint and review-round ids from the code

The code cited its plan instead of saying why: `E28 review J4`, `(R6 S3b)`, `S1 justified…`,
`deep-review round 5`. That plan lives in the private ops repo, so for every reader of this one the
citation was noise, and a sentence whose subject was the plan step ("round 2 showed…") hid its
lesson behind an id. Each one is gone and the "why" kept: a bare citation is deleted, a sentence
carrying a lesson is reworded around it ("a later attempt showed…").

Measured (`scripts/audit_metrics.py`, product code): 652 lines in 270 files carried an epic code by
the definition the next commit holds (507 / 241 by the old, two-digit-only one) -> 0. Across all
code, tests, scripts and configs: ~1100 epic-code lines plus ~320 sprint/round ids.

Behaviour unchanged, checked mechanically: every changed .py file has the same AST once docstrings
are blanked; css/sh/toml/yml/json files are identical once comments are stripped; the only
non-comment changes in TS/JS are test titles, one description string in a contract table, and five
ESLint messages, whose text is all the gates snapshot diff holds. Not touched: Markdown (docs and
the changelog keep their history), Alembic migrations, TV episode notation (`S2 · E5`), DESIGN.md
ids (`C1`, `B4`, `S3h`), tracker ids (`SEC-16`, `DX-17`).

One stale claim fixed on the way: test_hls_reaper's docstring said the player checkpoints "every
10 s whether or not it is playing", which plex/stream.py calls wrong; it now says what the code does.

frontend/src/components/PlexProvider.tsx

@@ -73,7 +73,7 @@ function readPlexSort(): SortState {
   return { key: raw, dir };
 }
 
-// S4: a shared Plex-browse link carries the content state in namespaced URL keys (pscope/psort/pshow/
+// A shared Plex-browse link carries the content state in namespaced URL keys (pscope/psort/pshow/
 // pf). When present, the URL wins over this account's stored view (like the feed's URL-over-localStorage
 // seed) — read once at mount. The keys are Plex-unique, so their mere presence identifies a shared Plex
 // view; no ?page check is needed (and the mirror below is page-gated so it never pollutes other pages).
@@ -194,7 +194,7 @@ export function PlexProvider({ children }: { children: ReactNode }) {
     [setScope, setShow, setSort, setFilters, setQ, setField],
   );
 
-  // S4: mirror the content state (scope + sort + watch-state + filters) into the address bar under the
+  // Mirror the content state (scope + sort + watch-state + filters) into the address bar under the
   // namespaced keys so a Plex-browse view is shareable. Gated on page===plex: off the Plex page the keys
   // are cleared (localStorage still holds the state; it re-mirrors on return). `q`/`field` stay OUT — the
   // search term is deliberately transient-per-view and not part of the shared four-key contract.