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/PlexCollectionView.tsx

@@ -282,7 +282,7 @@ export default function PlexCollectionView({
             posterDisabled={!canPlay || queueBusy}
             showPlayOverlay={canPlay}
             onPosterClick={() => canPlay && !queueBusy && playAll(false)}
-            // Sized to the text beside it (E31, the owner's pick): a title, one meta line and a short
+            // Sized to the text beside it (the owner's pick): a title, one meta line and a short
             // description sat beside a show-sized poster, two thirds of the hero empty under them.
             posterSizeClass="w-20 sm:w-24"
             customizeMenu={null}
@@ -423,7 +423,7 @@ export default function PlexCollectionView({
                 </div>
               </div>
               {/* Two columns on a phone, like every other poster grid (≥140px a card): three left
-                  an ~85px card whose title shared its line with the touch "…" (E28 C1). */}
+                  an ~85px card whose title shared its line with the touch "…". */}
               <NavGrid className="grid grid-cols-2 gap-3 sm:grid-cols-4 md:grid-cols-5 lg:grid-cols-6">
                 {items.map((c, i) => (
                   <PlexPosterCard
@@ -451,7 +451,7 @@ export default function PlexCollectionView({
           canEdit
             ? [
                 {
-                  // A write to the shared Plex library: a menu row since E28 C1 (the card's hover
+                  // A write to the shared Plex library: a menu row (the card's hover
                   // row owns its corner now), still behind the confirmation in `doRemove`.
                   id: "collection-remove",
                   label: t("plex.collection.removeItem"),