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/useVideoActions.tsx

@@ -26,7 +26,7 @@ import { useNavigationActions } from "./navigationContext";
 import { type ActionMenuItem } from "./ui/ActionMenu";
 import { VideoFollowUp, type VideoFollowUpState } from "./VideoFollowUp";
 
-// E28 C1 — a video's ONE action list (docs/DESIGN.md "Action placement"). The right-click, the
+// A video's ONE action list (docs/DESIGN.md "Action placement"). The right-click, the
 // long-press, Shift+F10 and the card's "…" all open a menu built here, so the feed, a channel page
 // and the Home shelves list the same actions in the same order. The card itself keeps only its
 // state and the everyday toggles (watched, save, add to playlist) on hover; everything else —
@@ -71,8 +71,8 @@ export function useVideoActions(handlers: VideoActionHandlers, open: boolean) {
     const inProgress = v.position_seconds > 0 && !watched;
     const dl = index.data?.[v.id];
     // The row opens the download dialog, so it says so; the download's state is the line under its
-    // label — "Downloaded" read as a status, not as a row that opens something (E28 review J4) — and
-    // not a `title`, which a key or a finger never shows (round 2, F5).
+    // label — "Downloaded" read as a status, not as a row that opens something — and
+    // not a `title`, which a key or a finger never shows.
     const dlState =
       dl === "done"
         ? t("downloads.button.downloaded")