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/AdminUsers.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// A failed user action is said once, and a user who is gone is not left listed (deep-review round 4).
+// A failed user action is said once, and a user who is gone is not left listed.
 // setRole / suspend / delete had no onError at all — "the global error dialog says it" — but a 404
 // (the user was deleted in another tab) gets no dialog, so the click did nothing and the row stayed.
 // And the invite approve/deny toasted unconditionally on top of the dialog `req` opens for a 409.
@@ -76,7 +76,7 @@ function mount() {
   return users;
 }
 
-describe("the last sign-in column (E22 S1)", () => {
+describe("the last sign-in column", () => {
   it("says how long ago each account signed in, and a dash where nothing is recorded", async () => {
     const at = new Date(Date.now() - 3 * 86_400_000).toISOString();
     vi.spyOn(api, "adminUsers").mockResolvedValue([
@@ -96,7 +96,7 @@ describe("the last sign-in column (E22 S1)", () => {
   });
 });
 
-// E28 (owner): the hover ended wherever the row's first badge or button began, because the layer was
+// The hover ended wherever the row's first badge or button began, because the layer was
 // the name button's own. The card is the row: it paints the layer under its content, and the name
 // button's hit layer spans the card, under the actions that sit above it.
 describe("a user row", () => {
@@ -117,7 +117,7 @@ describe("a user row", () => {
 /** The open menu's rows (a closed popover's are hidden from the accessibility tree in jsdom). */
 const menuRows = () => screen.queryAllByRole("menuitem", { hidden: true });
 
-/** The row's actions live in its "…" menu (E31): open it, then pick the row. */
+/** The row's actions live in its "…" menu: open it, then pick the row. */
 async function chooseInRowMenu(label: string) {
   fireEvent.click(await screen.findByRole("button", { name: "users.rowMenu" }));
   const item = await waitFor(() => {
@@ -128,7 +128,7 @@ async function chooseInRowMenu(label: string) {
   fireEvent.click(item);
 }
 
-// E31 (DESIGN § Action placement): the role, the suspension and the delete moved off the row into
+// DESIGN § Action placement: the role, the suspension and the delete moved off the row into
 // ONE menu — the row's "…", which a right-click on the row opens too. A locked one stays listed with
 // its reason.
 describe("a user row's menu", () => {