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/WriteFailureReporting.dom.test.tsx

@@ -2,10 +2,10 @@
 //
 // A write that fails is named, wherever it fails. `req`/`uploadReq` deliberately leave 403/404/429
 // to the caller (see reportFailure) and answer a 502/503/504 with only the transient "connection
-// lost" notice — so a handler with no `catch` reports nothing at all. Round 4 swept a dozen such
+// lost" notice — so a handler with no `catch` reports nothing at all. A sweep fixed a dozen such
 // call sites and missed these two: the collection editor's three siblings, in the very file it
 // edited, and the one `uploadReq` caller, whose empty catch still claimed the global error modal had
-// it (deep-review round 5).
+// it.
 import { describe, it, expect, vi, beforeAll, beforeEach, afterEach } from "vitest";
 import { render, cleanup, screen, fireEvent, waitFor } from "@testing-library/react";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
@@ -49,7 +49,7 @@ afterEach(() => {
   vi.restoreAllMocks();
 });
 
-// What failed, as the toast names it: the title (E11 S2b — the title says WHAT, the line under it WHY).
+// What failed, as the toast names it: the title (the title says WHAT, the line under it WHY).
 const errors = () =>
   notify.mock.calls.filter(([n]) => n.level === "error").map(([n]) => n.title ?? n.message);
 const client = () => new QueryClient({ defaultOptions: { queries: { retry: false } } });
@@ -131,7 +131,7 @@ describe("the upload dialog", () => {
     // fails, retries once after 250 ms and then raises the "connection lost" notice — a delayed
     // toast that outlives the test that spawned it and lands as an unexpected error in whichever
     // test is running by then. The file is fast enough today that it always lands after the suite,
-    // which is a coincidence, not a guarantee (deep-review round 2).
+    // which is a coincidence, not a guarantee.
     vi.spyOn(api, "downloadUsage").mockResolvedValue({
       upload_max_bytes: 1_000_000,
     } as Awaited<ReturnType<typeof api.downloadUsage>>);
@@ -162,7 +162,7 @@ describe("the upload dialog", () => {
 describe("the download dialog", () => {
   it("names a queueing the server refused with no dialog of its own", async () => {
     // Its catch said "the global error dialog already surfaced the reason" — true for a quota 422,
-    // not for a 403/404/429 or a lost connection, where the dialog just stayed open (E11 S2b).
+    // not for a 403/404/429 or a lost connection, where the dialog just stayed open.
     vi.spyOn(api, "downloadProfiles").mockResolvedValue([{ id: 1, name: "Best" }] as never);
     vi.spyOn(api, "enqueueDownload").mockRejectedValue(new HttpError(403, "nope"));
     render(