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/useUndoable.ts

@@ -13,7 +13,7 @@ import { useLatest } from "./useLatest";
 // of filters…) can use it. The history is state (what render reads) mirrored in a ref (what the
 // stable callbacks read, so two calls in one tick chain on each other, never on a stale snapshot —
 // save after a `reset`, which is state only and reaches the mirror at its commit; see `reset`).
-// Render never reads the ref: a compiled component would otherwise memoize a stale value (E32).
+// Render never reads the ref: a compiled component would otherwise memoize a stale value.
 export interface Undoable<T> {
   value: T;
   set: (next: T) => void;
@@ -98,7 +98,7 @@ export function useUndoable<T>(
   // still held the history before it, and rebasing that wrote the old value back over the reset (a
   // cached playlist remounted under StrictMode re-seeds with reset-then-rebase and painted empty).
   // The mirror is rebased too, so an `undo` later in the same tick steps back to a rebased snapshot,
-  // not to one that still holds the removed row (E32 review round 2). Called from handlers and
+  // not to one that still holds the removed row. Called from handlers and
   // effects only, never during render.
   const rebase = useCallback((map: (value: T) => T) => {
     const over = (s: History<T>) => ({