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/lib/nextPage.wiring.contract.test.ts

@@ -1,7 +1,7 @@
 import { describe, expect, it } from "vitest";
 
 // Every infinite list's read-ahead waits while the list fetches ANYTHING and runs again when the fetch
-// settles (E31 S5). A read-ahead that only checks `isFetchingNextPage` calls during a background
+// settles. A read-ahead that only checks `isFetchingNextPage` calls during a background
 // refetch, joins it through NEXT_PAGE and loads no page: the grid stalls at its loaded end until the
 // reader scrolls the range. The jsdom virtualizer measures nothing, so the wiring is pinned in source.
 const files = import.meta.glob(["../components/**/*.tsx", "!../components/**/*.test.tsx"], {
@@ -31,7 +31,7 @@ describe("an infinite list's read-ahead", () => {
   });
 
   // Retry is off, so a failed page settles at once; a read-ahead that re-runs on the settle would
-  // request it again once a round-trip while the end is in view (E31 review 2).
+  // request it again once a round-trip while the end is in view.
   it("stands down at a failed next page", () => {
     expect(source("Feed.tsx")).toMatch(
       /hasNextPage=\{!!hasNextPage && !query\.isFetchNextPageError\}/,
@@ -44,7 +44,7 @@ describe("an infinite list's read-ahead", () => {
 
 // A click joins an in-flight next page but cancels a background refetch (nextPageOnClick): through
 // NEXT_PAGE it would join the refetch and load nothing. NEXT_PAGE is only for the read-aheads, so a
-// click site that goes back to it fails here (E31 review 2: the YouTube "Load more" was missed).
+// click site that goes back to it fails here (the YouTube "Load more" was once missed).
 describe("a click that loads the next page", () => {
   const READ_AHEADS = new Set(["ui/VirtualGrid.tsx", "PlexBrowse.tsx"]);
   it("uses nextPageOnClick; NEXT_PAGE only in the read-aheads", () => {