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/api/plex.ts

@@ -55,8 +55,8 @@ export interface PlexCard {
   show_id?: string | null; // episode: the show's rating_key (for grouping a playlist by show/season)
   summary?: string | null; // movie/show hover-overview synopsis (trimmed) + episode summary
 }
-// TMDB enrichment block (E5 S2), served from OUR DB (never a live TMDB call) on a movie/show detail.
-// null until the row has been enriched. Powers the S5 enriched detail page: multi-source ratings row,
+// TMDB enrichment block, served from OUR DB (never a live TMDB call) on a movie/show detail.
+// null until the row has been enriched. Powers the enriched detail page: multi-source ratings row,
 // facts sidebar, keyword chips (→ filter), and the full crew.
 interface PlexTmdbCrew {
   name: string;
@@ -73,7 +73,7 @@ export interface PlexTmdb {
   keywords: string[];
   crew: PlexTmdbCrew[];
   studios: string[];
-  // ISO code (null on a row enriched before E30 whose name TMDB's list lacked) + TMDB's English name.
+  // ISO code (null on an older enriched row whose name TMDB's list lacked) + TMDB's English name.
   production_countries: { code: string | null; name: string }[];
   recommendations: { tmdb_id?: number | null; title?: string | null; in_library: boolean }[];
   backdrops: string[];
@@ -133,7 +133,7 @@ export interface PlexMarker {
   start_s: number;
   end_s: number;
 }
-// What the player names an audio/subtitle track from (E30, `lib/vocab` trackLabel). Plex's own title
+// What the player names an audio/subtitle track from (`lib/vocab` trackLabel). Plex's own title
 // (`label`) is in the Plex SERVER's language, so it is only the fallback for a track with no language tag.
 export interface PlexTrackNaming {
   ord: number;
@@ -239,7 +239,7 @@ export interface PlexPersonCard {
 export interface PlexPersonDetail {
   name: string;
   photo?: string | null; // cached headshot, or null (placeholder)
-  // TMDB bio (E5 S6), fetched ahead of time by the people_enrich job (E30 S6) and served from the row.
+  // TMDB bio, fetched ahead of time by the people_enrich job and served from the row.
   // Null when the person has no linked tmdb_person_id (never appeared in a TMDB-enriched title), TMDB
   // has none, or it is still on its way (`enriching`).
   biography?: string | null;
@@ -270,7 +270,7 @@ export interface PlexFilters {
   actorMode?: "any" | "all"; // how multiple actors combine (default any = OR)
   studios: string[]; // OR — from any selected studio
   playable: string[]; // OR — playback class (direct | remux | transcode_hw | transcode_sw); movie-only
-  keywords: string[]; // AND — TMDB keyword chips from the detail page (E5 S5)
+  keywords: string[]; // AND — TMDB keyword chips from the detail page
   collections: string[]; // Plex collection rating_keys; a title can be in several
   collectionMode?: "any" | "all"; // how multiple collections combine (default any = OR)
   collectionTitles?: Record<string, string>; // rating_key → title, for the active-filter chips
@@ -400,7 +400,7 @@ export function parsePlexFilters(p: URLSearchParams): PlexFilters {
 export interface PlexFacets {
   genres: { value: string; count: number }[];
   content_ratings: { value: string; count: number }[];
-  studios: { value: string; count: number }[]; // studio vocabulary for the sidebar picker (E5 S7)
+  studios: { value: string; count: number }[]; // studio vocabulary for the sidebar picker
   // Playback-class facet (movie-only): value ∈ direct|remux|transcode_hw|transcode_sw. Empty on shows.
   playable: { value: string; count: number }[];
   year_min: number | null;
@@ -418,7 +418,7 @@ export interface PlexPlaySession {
   tonemap?: boolean; // hls HW transcode only: 10-bit/HDR source tone-mapped HDR→SDR on the GPU (OpenCL)
 }
 
-// Discover surface (E5 S4): the consolidated rail payload. Every rail is a plain PlexCard list the
+// Discover surface: the consolidated rail payload. Every rail is a plain PlexCard list the
 // dedicated Discover landing renders as a horizontal Carousel; `because_you_watched` is null when the
 // user has no TMDB-enriched watch history (rail hidden). Served from DB — no live Plex/TMDB call.
 export interface PlexDiscover {