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/plexImage.contract.test.ts

@@ -5,8 +5,8 @@ import { cardImage } from "./plexImage";
 
 // A card's srcset names each Plex cut by its width, and the browser picks one by that width. The
 // widths are the image proxy's (backend `app/plex/imgcache.py` SIZES), with no wire format between
-// the two: a backend-only change would have the browser pick a cut by a width it does not have (E31
-// S5). Read from source, as plexFinishMargin.contract does.
+// the two: a backend-only change would have the browser pick a cut by a width it does not have.
+// Read from source, as plexFinishMargin.contract does.
 const source = readFileSync(
   new URL("../../../backend/app/plex/imgcache.py", import.meta.url),
   "utf8",
@@ -45,12 +45,12 @@ it("names each backdrop cut by the width the proxy cuts it at", () => {
   });
 });
 
-// Every small Plex picture asks for its card-size cut through cardImage (E31 S4e, swept in S5: the
+// Every small Plex picture asks for its card-size cut through cardImage (the
 // playlist rows, the search box and the collection picker still fetched the 400px poster for a 32px
 // picture). A plain `src={…}` is allowed only where the full cut is the point or the URL is not the
 // proxy's; the SET is pinned, so a new one fails here. Every <img>'s src counts, wherever it sits
 // among the attributes and whatever its variable is called: the first cut of this test read only a
-// leading `src={…thumb…}` and missed two of the three sites it was written for (E31 review 2).
+// leading `src={…thumb…}` and missed two of the three sites it was written for.
 const plexFiles = import.meta.glob(
   [
     "../components/{Plex,plex}*.tsx",