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/PlexPlayer.session.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// The Plex player's session lifecycle, rendered for real (deep-review round 3 of bug hunt 2026-09).
+// The Plex player's session lifecycle, rendered for real.
 //
 // jsdom has no media pipeline, so the <video> is a small stand-in that behaves like Chrome where it
 // matters here: `pause()` fires its `timeupdate` in a LATER task (measured in Chrome — which is how the
@@ -279,7 +279,7 @@ it("shows a toast raised while it is open inside itself, not under its scrim", a
   expect(host).not.toBe(document.body); // the player's own host, beside its <video>
 });
 
-// --- deep-review round 4 ------------------------------------------------------------------------
+// --- leaving a played-through item, restarts, failed requests -----------------------------------
 
 /** A session answer that FAILS when the test says so. */
 function deferredFailure() {
@@ -300,8 +300,7 @@ describe("leaving an item that was played through", () => {
 
   // The end of the media, in the order the HTML spec mandates: the browser sets `paused`, fires
   // 'pause', and only THEN fires 'ended'. Dispatching a bare 'ended' models a browser that does not
-  // exist, and hid the fact that the pause checkpoint goes out before the finish latch is up
-  // (deep-review round 5).
+  // exist, and hid the fact that the pause checkpoint goes out before the finish latch is up.
   const endMedia = () => {
     const el = video();
     Object.defineProperty(el, "ended", { configurable: true, get: () => true });
@@ -361,7 +360,7 @@ describe("leaving an item that was played through", () => {
     // The mark was fired and forgotten (`.catch(() => {})`) while the cache was patched to
     // watched / position 0 regardless, and the once-per-id start guard meant the corrected detail
     // arriving later was never used: the episode reopened at 0:00 with a Watched badge the server
-    // disagreed with (round 5).
+    // disagreed with.
     prefs({ upNext: false });
     const { qc, progress } = mount({
       details: { A: item("A"), B: item("B") },
@@ -379,19 +378,19 @@ describe("leaving an item that was played through", () => {
     // And the rollback does NOT write the position back. 2995 of 3000 is inside the backend's
     // finish margin, so that checkpoint would have set watched / position 0 and scrobbled to Plex
     // — performing, through a second route, the very write that was just refused, while the error
-    // toast said it had failed (round 6). There is no resume point at the end of the media to
+    // toast said it had failed. There is no resume point at the end of the media to
     // restore; the cache drop is the whole of the rollback here.
     expect(progress.mock.calls.filter(([id, , , final]) => id === "A" && final)).toEqual([]);
   });
 
   it("performs the refused mark through no OTHER route while it stays on the finished item", async () => {
-    // Round 6 guarded the explicit rollback write, but the same catch lowered the play-through
+    // A fix guarded the explicit rollback write, but the same catch lowered the play-through
     // latch — and that latch is the only gate on the 10s interval, the page-hide beacon and
     // `settle`. All three post `absRef`, which on this path IS the end of the media, and the
     // backend reads a position within `_FINISH_MARGIN_S` of the duration as a finish: within ten
     // seconds of the toast saying the mark had failed, the item was watched, its position cleared
-    // and a scrobble sent to Plex (deep-review round 7). `upNext` is ON here — the default — so the
-    // player STAYS on the finished item and those routes are live; the round-6 test above turns it
+    // and a scrobble sent to Plex. `upNext` is ON here — the default — so the
+    // player STAYS on the finished item and those routes are live; the test above turns it
     // off, which tears the effect down before any of them can fire.
     prefs({ upNext: true });
     const beacon = vi.spyOn(api, "plexProgressBeacon").mockImplementation(() => {});
@@ -467,7 +466,7 @@ describe("leaving an item that was played through", () => {
   it("records a second viewing when the viewer plays on from the finished frame", async () => {
     // `finished` is a latch that makes every checkpoint a no-op; the only thing that lowers it is
     // the video's own 'play' event. Nothing tested that, so dropping the listener would have
-    // discarded a whole rewatch with the gate green (round 5).
+    // discarded a whole rewatch with the gate green.
     prefs({ upNext: true });
     const { progress } = mount({
       details: { A: item("A"), B: item("B") },