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/comfortPin.tsx

@@ -10,13 +10,13 @@ import { notify } from "../lib/notifications";
 import { qk } from "../lib/queryKeys";
 import { type ActionMenuItem } from "./ui/ActionMenu";
 
-// E24 S3 — "put this on my comfort shelf", from wherever you happen to be looking at the thing.
+// "put this on my comfort shelf", from wherever you happen to be looking at the thing.
 //
 // The shelf is the Home surface's object, but the GESTURE belongs wherever a pinnable thing is
 // shown — a video, a channel, a playlist, a Plex show or movie: you decide something is comfort media
 // while you are watching it. (Locked decision 3: an episode, a season and a person are not pin kinds,
 // so their cards and strips carry no pin.) This file is the one place the pin lives; every surface
-// composes the same `ActionMenuItem` into its object's ONE action list (E28 C1): the video, Plex and
+// composes the same `ActionMenuItem` into its object's ONE action list: the video, Plex and
 // channel actions (`useVideoActions`, `usePlexCardActions`, `Channels`), the playlist rail and page,
 // and Home's shelves. There is no pin-only menu any more.
 //
@@ -199,7 +199,7 @@ export function useComfortPin(open: boolean): ComfortPin {
           // label and the icon rather than being decided at a call site. The shelf's own hand-rolled
           // unpin row on Home got the flag first; leaving this one neutral painted the identical
           // command (same label, same icon, same `removeComfortPin`) two different colours on one
-          // screen — the hero's menu and the Continue-watching rail come through here (round 2).
+          // screen — the hero's menu and the Continue-watching rail come through here.
           danger: pinId !== null,
           onSelect: () => toggle(target, title),
         },