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/PlayerModal.tsx

@@ -100,7 +100,7 @@ function canScrollBy(el: HTMLElement | null, dir: -1 | 1): boolean {
   return false;
 }
 
-// From this window width the player's details sit in a side panel beside the video (E31). The panel is
+// From this window width the player's details sit in a side panel beside the video. The panel is
 // a rem width, so it grows with the text size; the video takes what is left.
 const WIDE_PLAYER_PX = 1280;
 const SIDE_PANEL = "20rem";
@@ -206,10 +206,10 @@ export default function PlayerModal({
   const focusModal = useCallback(() => cardRef.current?.focus(), []);
   // The card fits the window, video AND what sits under it (the queue strip, title, channel row): the
   // video is 16:9 at the card's width, so on a short window (133% zoom on a 1080p screen: 685px) a
-  // full-width card ran past the bottom and cut its channel row (E31 S1). `below` is that chrome's
+  // full-width card ran past the bottom and cut its channel row. `below` is that chrome's
   // height, measured (it wraps with the title and the text size), and caps the card's width below.
   const [below, setBelow] = useState(0);
-  // A wide window (E31, the owner's pick "A"): the title, channel, actions, description and the queue
+  // A wide window (the owner's pick): the title, channel, actions, description and the queue
   // move into a side panel beside the video, so the video takes the window's whole height — at
   // 1920×911 it was capped at ~1008 px wide with empty space all round. Narrower, they stay under it.
   const wide = useMediaQuery(`(min-width: ${WIDE_PLAYER_PX}px)`, false);
@@ -229,8 +229,8 @@ export default function PlayerModal({
   }, []);
   // The dialog contract every other `aria-modal` shell shares (ui/dialogFocus): the focus goes back
   // to whatever opened the player, and the page lock is the one counted lock. It opens focused on the
-  // CARD, not its first button, so Space/F/arrows reach the shortcuts. Until E11 this was the one
-  // hand-rolled shell — no accessible name, no Tab trap, no restore (deep-review round 7).
+  // CARD, not its first button, so Space/F/arrows reach the shortcuts. This was once the one
+  // hand-rolled shell — no accessible name, no Tab trap, no restore.
   useDialogChrome(cardRef, dialogRef, { focusScope: true });
   const titleId = useId();
   const togglePlay = () => {
@@ -477,7 +477,7 @@ export default function PlayerModal({
   // over the player by doing so.
   const overlayOrder = useOverlayOrder();
   // This player is a dismissable layer: register it so a Modal or popover opened ABOVE it (e.g. the
-  // download dialog, or — after S4 — an anchored menu) takes Escape first and this player defers.
+  // download dialog, or an anchored menu) takes Escape first and this player defers.
   const layerIdRef = useRef<number | null>(null);
   useEffect(() => {
     const id = pushLayer();