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.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// E28 review J4: the video menu's download row was labelled with the download's STATE ("Downloaded",
+// The video menu's download row was labelled with the download's STATE ("Downloaded",
 // "In queue") — a row that reads as a status yet opens the download dialog. It is "Download…" now,
 // the state riding along as the row's hint.
 import type { ReactNode } from "react";
@@ -44,7 +44,7 @@ function downloadRow(state: string | undefined) {
 }
 
 describe("the video menu's download row", () => {
-  // Round 2 (F5): the state rode in a `title`, which a key or a finger never shows.
+  // The state rode in a `title`, which a key or a finger never shows.
   it("says what it does, whatever the state, and carries the state as its note", async () => {
     const done = downloadRow("done");
     await waitFor(() => expect(done().note).toBe("downloads.button.downloaded"));