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/Chip.tsx

@@ -58,7 +58,7 @@ export const Chip = forwardRef<HTMLButtonElement, ChipProps>(function Chip(
       className={clsx(chipClass(variant, size), removeLabel && "pr-1.5", className)}
       onClick={(e) => {
         // A removal chip unmounts with what it removes: from the keyboard it holds the focus, which
-        // went to <body> (round 2, F6 — the active-filter chips of the feed and Plex). Every
+        // went to <body> (the active-filter chips of the feed and Plex). Every
         // removal chip gets it here, once, instead of each caller remembering.
         if (removeLabel && e.currentTarget === document.activeElement)
           handOffFocus(e.currentTarget, () => onClick?.(e));
@@ -126,8 +126,8 @@ export function DismissibleChip({
         type="button"
         onClick={(e) => {
           // From the keyboard the × holds the focus, and the chip unmounts: hand the focus to the
-          // next chip (else the previous one, else what follows the row) first, not to <body> (E28
-          // review F5). A mouse press on it leaves the focus where it was.
+          // next chip (else the previous one, else what follows the row) first, not to <body>.
+          // A mouse press on it leaves the focus where it was.
           const chip = e.currentTarget.parentElement;
           if (chip && e.currentTarget === document.activeElement) handOffFocus(chip, onDismiss);
           else onDismiss();