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/feed.ts

@@ -99,7 +99,7 @@ export interface FeedFilters {
   year?: number;
 }
 
-/** One year's video count for the year control's chips + "more…" dialog (item 8 / E8·S2). */
+/** One year's video count for the year control's chips + "more…" dialog. */
 export interface YearCount {
   year: number;
   count: number;
@@ -169,7 +169,7 @@ export const feedApi = {
   // the separate feedCount + facets the feed and sidebar used to fire on every filter change (PERF-13).
   feedSummary: (f: FeedFilters): Promise<{ count: number; facets: Record<string, number> }> =>
     req(`/api/feed/summary?${filterParams(f).toString()}`),
-  // Per-year video counts for the year control (item 8 / E8·S2). The backend drops the publish-date
+  // Per-year video counts for the year control. The backend drops the publish-date
   // narrowing, so callers key this on the date-stripped filters (picking a year mustn't refetch it).
   feedYears: (f: FeedFilters): Promise<{ years: YearCount[] }> =>
     req(`/api/feed/years?${filterParams(f).toString()}`),