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/playerModalHooks.ts

@@ -155,7 +155,7 @@ export function useYtVolume({
     // Ctrl/⌘+wheel is the browser's zoom gesture — never swallow it.
     if (e.ctrlKey || e.metaKey) return;
     // Over a panel that can still scroll that way, the wheel scrolls it: the wide player's side panel
-    // holds the description, which a wheel spent on the volume could never be read past (E31).
+    // holds the description, which a wheel spent on the volume could never be read past.
     const panel =
       e.target instanceof Element ? e.target.closest<HTMLElement>("[data-wheel-scrolls]") : null;
     if (
@@ -555,7 +555,7 @@ export function useYtProgress({
     // this cleanup's late auto-mark land on the item being LEFT, which the modal then ignores. What
     // genuinely has to stay live mid-session is behind refs (currentIdRef, modeRef, navRef,
     // sessionPosRef, autoMarkedRef, errorStreakRef, playerRef) — and THAT split is the enforcement:
-    // a value that is not a snapshot does not belong in this closure. (E32: this replaced an
+    // a value that is not a snapshot does not belong in this closure. (This replaced an
     // exhaustive-deps suppression, which made the React Compiler skip the whole hook.)
   }, [activeId, mountRef, playerRef, currentIdRef, navRef, modeRef, sessionPosRef]);
 }