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/PlexPersonView.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// E5 S4: the actor detail page. Covers the two pieces the /code-review flagged as untested — the
+// The actor detail page. Covers the two pieces the /code-review flagged as untested — the
 // cast-row rewiring (a cast click now OPENS the person page via onOpenPerson, falling back to the old
 // filter gesture only when no onOpenPerson is wired) and PlexPersonView's render (headshot/name hero,
 // the Movies/Shows sections, and the "filter library by this person" action). i18n is key-passthrough
@@ -117,11 +117,11 @@ describe("PlexPersonView", () => {
     expect(screen.getByText("plex.person.movies")).toBeTruthy(); // section headings (key passthrough)
     expect(screen.getByText("plex.person.shows")).toBeTruthy();
 
-    // E5 S6 bio hero: department, biography, birth facts, aka.
-    // E30: TMDB's English department, named through the vocab (key-echo i18n here).
+    // The bio hero: department, biography, birth facts, aka.
+    // TMDB's English department, named through the vocab (key-echo i18n here).
     expect(screen.getByText("vocab.department.Acting")).toBeTruthy();
     expect(screen.getByText("A celebrated actor.")).toBeTruthy();
-    // E30 S3: the birth date as a date in the UI language, not the raw ISO string
+    // The birth date as a date in the UI language, not the raw ISO string
     expect(screen.getByText("Jan 1, 1970 · Springfield")).toBeTruthy();
     expect(screen.getByText(/plex\.person\.aka/)).toBeTruthy();
     expect(screen.getByText(/Janie D/)).toBeTruthy();
@@ -157,7 +157,7 @@ describe("PlexPersonView", () => {
   });
 });
 
-describe("PlexPersonView — the reader's language (E30)", () => {
+describe("PlexPersonView — the reader's language", () => {
   it("asks for the filmography in Hungarian", async () => {
     setLanguage("hu");
     try {
@@ -189,7 +189,7 @@ describe("PlexPersonView — the reader's language (E30)", () => {
 
 const BIO_POLL = 1600; // just past plexViews' BIO_POLL_MS
 
-describe("PlexPersonView — a bio on its way (E30 S6)", () => {
+describe("PlexPersonView — a bio on its way", () => {
   const view = (
     <PlexPersonView
       name="Jane Doe"
@@ -320,7 +320,7 @@ describe("PlexPersonView — a bio on its way (E30 S6)", () => {
   });
 });
 
-describe("PlexPersonView — the hero's columns (E30 S6)", () => {
+describe("PlexPersonView — the hero's columns", () => {
   const heroGrid = async (knownFor: number) => {
     plexPerson.mockReset();
     plexPerson.mockResolvedValue({