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/VideoCard.tsx

@@ -47,7 +47,7 @@ import { RelativeTime } from "./ui/RelativeTime";
 /** The "…" that opens the video's action list, and whether its menu is the open one. */
 type More = { props: Record<string, unknown>; open: boolean };
 
-// E28 C1: the card keeps its state and the everyday toggles — watched, save, add to playlist — and
+// The card keeps its state and the everyday toggles — watched, save, add to playlist — and
 // a "…" for everything else (download, details, copy, hide, reset, pin), which is the list's ONE
 // menu (useVideoActions). On a mouse the row shows on hover or keyboard focus; on touch the toggles
 // are gone (the "…" and the long-press are the way in) and only the "…" stays.
@@ -242,7 +242,7 @@ function Thumb({
     // tall, which stretched a whole Home shelf to it (ui/mediaCardStyles `mediaPictureBox`).
     <div className={clsx("relative min-h-0", MEDIA_PICTURE_CONTAINER, className)}>
       {/* Named by the title: its only text is the duration badge, so a screen reader heard the first
-        Tab stop of every card as "44:10" (E28 S3d4). */}
+        Tab stop of every card as "44:10". */}
       <a
         href={video.watch_url}
         target="_blank"
@@ -381,7 +381,7 @@ function VideoCard({
   onState: (id: string, status: VideoStatus) => void;
   onToggleSave: (id: string, saved: boolean) => void;
   onOpen?: (v: Video, startAt?: number | null) => void;
-  /** Right-click / long-press props for the LIST's one context menu (E28 C1: the video's actions).
+  /** Right-click / long-press props for the LIST's one context menu (the video's actions).
    *  A FUNCTION, not the already-built props object: `useContextMenu.triggerProps` is stable across
    *  renders while its return value is a fresh object every call, and a fresh object prop would
    *  blow past this component's memo for every card in the virtualized feed on every parent
@@ -460,7 +460,7 @@ function VideoCard({
       rel="noreferrer"
       onClick={(e) => openInApp(e, video, onOpen)}
       // A link is dragged, not selected: Chrome starts no selection on one even with the drag off,
-      // until it is also `user-select: text` (select-text below) — measured in the S3e1 UAT.
+      // until it is also `user-select: text` (select-text below) — measured in a real browser.
       draggable={false}
       // Enter on a keyboard-selected card clicks THIS link (lib/listNav), so it opens exactly like a
       // click: the in-app player, resuming where the reader left off.
@@ -544,7 +544,7 @@ function VideoCard({
           {...(navIndex !== undefined && navItemAttrs(navIndex))}
           className={clsx(rowShell, "flex items-center gap-3 px-3 py-2", watched && "opacity-55")}
         >
-          {/* The title leads (E28 S3h, the owner's pick B, 2026-09-26): the actions used to lead in a
+          {/* The title leads (the owner's pick, 2026-09-26): the actions used to lead in a
               128px column that sat EMPTY at rest, a hole at the start of every row. Now the row's
               end holds a faint "…" (always there: the menu's door on every device), and on hover or
               keyboard focus the quick toggles float in beside it OVER the date, which steps aside —