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/discover.ts

@@ -1,4 +1,4 @@
-// The Home surface ("Mit nézzek most?", E24): one hero suggestion, a cross-source resume rail, and
+// The Home surface ("Mit nézzek most?"): one hero suggestion, a cross-source resume rail, and
 // the comfort shelf. Types mirror `backend/app/discover/` — the card carries the NATIVE feed / Plex
 // payload under `video` / `plex`, so the page renders through the existing VideoCard /
 // PlexPosterCard instead of a third card component.
@@ -71,7 +71,7 @@ export interface DiscoverPayload {
 }
 
 export const discoverApi = {
-  // `lang` "hu" serves a Plex card's Hungarian title (E30); en is the server default.
+  // `lang` "hu" serves a Plex card's Hungarian title; en is the server default.
   discover: (budget: DiscoverBudget, seed: number, lang?: string): Promise<DiscoverPayload> =>
     req(`/api/discover?budget=${budget}&seed=${seed}${lang === "hu" ? "&lang=hu" : ""}`),
   // The "Mást kérek" reroll: a new hero alone, so pressing it does not re-fetch the rails under the