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/ChannelPage.tsx

@@ -90,7 +90,7 @@ export default function ChannelPage({
   // where the channelScoped Feed portals ITS toolbar (via the overridden PageToolbarSlot below), so
   // the toolbar lands under the tabs inside the fixed band rather than in the scroller.
   const [chromeSlot, setChromeSlot] = useState<HTMLDivElement | null>(null);
-  // The banner and the identity block fold away once the grid scrolls (E31, the owner's pick): the chrome
+  // The banner and the identity block fold away once the grid scrolls (owner's pick): the chrome
   // is fixed, so unfolded it kept ~40% of a 133% screen for good. Folded, the name rides in the tab row.
   const foldable = useRef<HTMLDivElement>(null);
   const folded = useScrollFold(foldable);
@@ -102,7 +102,7 @@ export default function ChannelPage({
   const MAX_BANNER_RETRIES = 3;
   const [bannerAttempt, setBannerAttempt] = useState(0);
   // The banner source that has actually loaded. Until then the <img> is invisible: an attempt that
-  // fails shows the browser's broken-image glyph for as long as the retry waits (seen live, E28).
+  // fails shows the browser's broken-image glyph for as long as the retry waits (seen live).
   const [bannerLoaded, setBannerLoaded] = useState<string | null>(null);
 
   // A LIVE query: while THIS user's full-history backfill is running, poll so the stored count (and
@@ -144,7 +144,7 @@ export default function ChannelPage({
             qc.invalidateQueries({ queryKey: qk.channel(channelId) });
           })
           // Both the automatic first-visit explore and "load more" used to fail in silence: an empty
-          // channel page that looked like a channel with no videos (E11 S2b).
+          // channel page that looked like a channel with no videos.
           .catch((err: unknown) => notifyActionError(err, t("errors.writeFailed.exploreChannel")))
           .finally(() => setExploring(false))
       );
@@ -267,7 +267,7 @@ export default function ChannelPage({
 
   // Channel-scoped feed: the whole catalog filtered to this channel (scope=all + source=all so
   // the explored, per-user-private videos show — the backend gates them to this explorer).
-  // The watch state is the one filter a channel page remembers (the owner, E31): a fresh page shows
+  // The watch state is the one filter a channel page remembers (owner's call): a fresh page shows
   // what is still unwatched, and a change sticks — across channels, reloads and devices.
   const [show, setShow] = usePref<string>("channelShow", "unwatched", {
     localKey: LS.channelShow,
@@ -313,7 +313,7 @@ export default function ChannelPage({
     ch?.total_view_count != null
       ? t("channel.totalViews", { formatted: formatCompact(ch.total_view_count, lang) })
       : null,
-    // Per-type breakdown (R14 moved this off the manager table's N/S/L column). Only meaningful
+    // Per-type breakdown (moved here off the manager table's old N/S/L column). Only meaningful
     // once we actually hold some of the channel's videos.
     ch && ch.stored_videos > 0
       ? t("channel.videoTypes", {
@@ -439,7 +439,7 @@ export default function ChannelPage({
             corner: there are only ever two tabs, so there's room, and it keeps the things you click
             within a few pixels of each other instead of across the page. */}
             {/* Wraps rather than overflows: at a tablet width (1024) the actions + tags ran past the edge and
-            the whole page scrolled sideways (E31 S1). The tag lane takes what is left and folds into "+N". */}
+            the whole page scrolled sideways. The tag lane takes what is left and folds into "+N". */}
             <div
               className={`flex flex-wrap items-center gap-2 border-b border-border pb-2 ${folded ? "pt-3" : "mt-3"}`}
             >