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/PlexPlaylistView.tsx

@@ -64,7 +64,7 @@ import { cardImage } from "../lib/plexImage";
 // per-account preference. Rendered as a PlexBrowse subview; "Play all" opens the player with the
 // whole ordered list as its queue.
 //
-// E28 C1: every row — a title, a show group, a season — has ONE menu, opened by its "…" (shown on
+// Every row — a title, a show group, a season — has ONE menu, opened by its "…" (shown on
 // hover or focus, always on touch), a right-click, a long-press or Shift+F10. Removing needs no
 // confirm: the toast's Undo puts the rows back where they stood. `j`/`k` walk the visible rows and
 // Delete takes out the one the keyboard is on; the playlist's own rename and delete sit in the
@@ -436,7 +436,7 @@ export default function PlexPlaylistView({
     usePlexPlaylistEdits(playlistId);
   const blocks = useMemo(() => buildBlocks(items), [items]);
   const order = useMemo(() => items.map((i) => i.id), [items]);
-  // Total playtime of the whole playlist, mirroring the YouTube playlist header (E9 12D): items with
+  // Total playtime of the whole playlist, mirroring the YouTube playlist header: items with
   // an unknown duration count as 0, and the runtime is dropped from the meta line when nothing sums.
   const totalSeconds = useMemo(
     () => items.reduce((acc, it) => acc + (it.duration_seconds ?? 0), 0),
@@ -553,7 +553,7 @@ export default function PlexPlaylistView({
     if (to >= 0) moveEpisode(aId, to);
   }
 
-  // --- The rows' ONE menu (E28 C1) -------------------------------------------------------------
+  // --- The rows' ONE menu -------------------------------------------------------------
   // Keyed: a row builds its target afresh each render, and its "…" must close the menu it opened.
   const rowMenu = useContextMenu<MenuTarget>({ keyOf: targetKey });
   const copy = useCopyItems();
@@ -780,7 +780,7 @@ export default function PlexPlaylistView({
               {t("plex.playlist.playAll")}
             </Button>
           )}
-          {/* Rename and delete — the playlist's own, rarer actions — sit in its "…" (E28 C1); a
+          {/* Rename and delete — the playlist's own, rarer actions — sit in its "…"; a
               rename opened from there hands the focus back to it. */}
           <MoreButton
             ref={renameTarget}