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/ui/ItemCount.tsx

@@ -8,7 +8,7 @@ import { formatNumber } from "../../lib/format";
 const OVERFLOW_AT = 1_000_000;
 
 // A compact, FIXED-WIDTH count pill for a toolbar: the filtered item count at the toolbar's 28, flat
-// and tonal like a `Badge` — no edge, because it does nothing (E28 S3d: edged = pressable, flat = just
+// and tonal like a `Badge` — no edge, because it does nothing (edged = pressable, flat = just
 // says; it used to wear the filter dropdowns' glass pill and read as one more). The fixed width is the point — a
 // changing number must never shove the controls beside it, so the box is sized to the widest value
 // ("999,999 items") and the content is centred within it. A count of a million or more collapses to
@@ -17,7 +17,7 @@ const OVERFLOW_AT = 1_000_000;
 // kept while the new one loads (keepPreviousData) — dimmed, and "updating" for a screen reader (which
 // does not voice aria-busy on plain text), so a broad search's slow count ("a": ~3 s) never reads as
 // the new result. `failed`: the count could not be read — a dash that says so, not the icon-only box
-// of a first load (E31 review 2).
+// of a first load.
 export default function ItemCount({
   count,
   stale = false,