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/PlexSearchBox.tsx

@@ -14,7 +14,7 @@ import { SURFACE } from "./ui/surfaces";
 import { ROVING_HIGHLIGHT } from "./ui/AnchoredPopover";
 import { cardImage } from "../lib/plexImage";
 
-// The Plex header search box: the shared SearchBar enriched with the S2 search scope-pill (a
+// The Plex header search box: the shared SearchBar enriched with the search scope-pill (a
 // GitHub-style field selector inside the box) and an instant typeahead of the top matches attached
 // below it. Kept out of the generic SearchBar (and out of Header) so all Plex-specific wiring —
 // provider state, the suggest query, the jump-to hand-off — lives in one place. Everything reuses
@@ -140,7 +140,7 @@ export default function PlexSearchBox() {
           <div
             className={`${SURFACE.menu} absolute left-0 right-0 top-full z-menu mt-2 max-h-[70dvh] overflow-y-auto animate-[popIn_0.16s_ease]`}
           >
-            {/* Beside the listbox, not in it: a listbox may own options only (round 2, F4). */}
+            {/* Beside the listbox, not in it: a listbox may own options only. */}
             {suggestions.length === 0 && suggestQ.isFetching && (
               <div className="px-3 py-2 text-sm text-muted">{t("plex.loading")}</div>
             )}