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/downloadRow.tsx

@@ -67,7 +67,7 @@ export function StatusPill({ job }: { job: DownloadJob }) {
 
 // The row answers to its OWN width (a container query), not the window's: the same list sits in a
 // phone, a split-screen tablet and a desktop column. Below 40rem the actions no longer leave the title
-// column room for the meta line, so the row stacks (E28 S3f3, the owner's pick A): the picture beside a
+// column room for the meta line, so the row stacks (the owner's pick A): the picture beside a
 // two-line title and the channel; the meta line across the whole row; then the source link and the
 // actions on one wrapping line.
 const THUMB_BOX =
@@ -95,7 +95,7 @@ function Thumb({ job }: { job: DownloadJob }) {
   const [failedPrimary, setFailedPrimary] = useState<string | null>(null);
   if (failedPrimary !== null && failedPrimary !== primary) setFailedPrimary(null);
   // The remote picture is the last resort, and it fails too (a dead CDN link): remembered the same
-  // way, so the row falls to the glyph below instead of the browser's broken-image mark (E28 C5).
+  // way, so the row falls to the glyph below instead of the browser's broken-image mark.
   const [failedRemote, setFailedRemote] = useState<string | null>(null);
   if (failedRemote !== null && failedRemote !== remote) setFailedRemote(null);
   const candidate = failedPrimary && failedPrimary === primary ? remote : primary;
@@ -175,7 +175,7 @@ export function IconBtn({
 }
 
 // A link inside the row keeps the BROWSER's own right-click menu (open in a new tab, copy the
-// address): the row's menu answers everywhere else on it (E28 C1).
+// address): the row's menu answers everywhere else on it.
 const nativeMenu = { onContextMenu: (e: React.MouseEvent) => e.stopPropagation() };
 
 // "Downloaded from" reference: opens the original page in a new tab. Its copy moved into the row's
@@ -236,7 +236,7 @@ export function DownloadRow({
   navIndex?: number;
   /** The list's menu triggers (right-click, long-press, Shift+F10): `useContextMenu().triggerProps`. */
   rowProps?: Record<string, unknown>;
-  /** The row's "…", last in its action strip (E28 C1). */
+  /** The row's "…", last in its action strip. */
   more?: ReactNode;
   children?: ReactNode;
 }) {