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/PlexInfo.tsx

@@ -38,7 +38,7 @@ import { vocab } from "../lib/vocab";
 // a card's "i" button (variant="page", with a big Play/Resume + watch controls), and a lean overlay
 // over the running player (variant="overlay", opened with "i"). Two GUI elements are user-toggleable
 // personal preferences, persisted in users.preferences: the faint art backdrop and the cast row.
-// The TMDB-enriched pieces (multi-source ratings row, facts, keyword chips, full crew — E5 S5) render
+// The TMDB-enriched pieces (multi-source ratings row, facts, keyword chips, full crew) render
 // on the page variant only; the overlay stays lean.
 
 // One rating chip's number: a filter button when this is the score the backend's ratingMin column
@@ -74,7 +74,7 @@ function Score({
   );
 }
 
-// Multi-source ratings cluster (E5 S5): IMDb (links out) · TMDb (with vote count) · Plex audience —
+// Multi-source ratings cluster: IMDb (links out) · TMDb (with vote count) · Plex audience —
 // whichever are present. The "primary" score (IMDb, else audience) mirrors the backend rating column
 // (coalesce(imdb, audience)), so only it carries the ratingMin filter; the others are informational.
 function RatingsRow({
@@ -181,8 +181,7 @@ type Props = {
   // When provided, a cast member's photo/name opens their filmography page instead of filtering.
   onOpenPerson?: (name: string) => void;
   // Play a sibling title from a collection strip: the page opens its player, the overlay plays it
-  // in the running one. Required, so no variant draws a collection card whose Play does nothing
-  // (E28 review D1).
+  // in the running one. Required, so no variant draws a collection card whose Play does nothing.
   onPlayItem: (id: string, fromStart?: boolean) => void;
   // The item's library key — enables the admin "Manage collections" editor (page variant, movies).
   library?: string;
@@ -229,7 +228,7 @@ export default function PlexInfo({
   const [playlistOpen, setPlaylistOpen] = useState(false);
 
   // Faint art backdrop, HTPC-style (page variant only) — cross-dissolves through the item's own art,
-  // its TMDB backdrops (E5 S2), and its collection siblings' art (E5 S8). Cleared in the overlay
+  // its TMDB backdrops, and its collection siblings' art. Cleared in the overlay
   // variant / when disabled / on unmount.
   const backdropPool = [
     detail.art,
@@ -251,7 +250,7 @@ export default function PlexInfo({
     onStateChange?.();
   };
 
-  // A collection sibling is a Plex card like any other (E28 review D2): the related strip's menu, its
+  // A collection sibling is a Plex card like any other: the related strip's menu, its
   // watched toggle and its playlist add. The flip lands on this item's cached detail, whose strip it
   // is; the resync corrects it to the truth either way.
   const colMenu = useContextMenu<PlexCard>();
@@ -416,7 +415,7 @@ export default function PlexInfo({
                   <PlaybackChip detail={detail} overlay={overlay} />
                 </div>
 
-                {/* Multi-source ratings row (E5 S5): IMDb / TMDb / audience, whichever are present. */}
+                {/* Multi-source ratings row: IMDb / TMDb / audience, whichever are present. */}
                 <div className="mt-2">
                   <RatingsRow
                     imdbRating={detail.imdb_rating}
@@ -539,7 +538,7 @@ export default function PlexInfo({
 
         {/* Below the hero: the collection strips first (the owner's most used), then the cast, each
             per-user toggleable, stacked so a hidden one leaves no hole. The facts / keywords / crew that
-            sat beside them (E28 S3h) moved up into the hero's second column (E31). The overlay (over
+            sat beside them moved up into the hero's second column. The overlay (over
             the running player) is the same stack, lean. */}
         {(() => {
           const collections = (detail.collections ?? []).filter(
@@ -568,7 +567,7 @@ export default function PlexInfo({
               </div>
               <Carousel ariaLabel={col.title} gap="gap-3" keyboardNav>
                 {col.items.map((m, i) => (
-                  // The one media card, as on every other strip (E28 S3h). A grid cell, so the
+                  // The one media card, as on every other strip. A grid cell, so the
                   // card fills the slot the carousel stretches to its tallest card.
                   <div key={m.id} className="grid w-37.5 shrink-0 grid-cols-1">
                     <PlexPosterCard