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/NavRail.tsx

@@ -92,7 +92,7 @@ export function NavRailItem({
   active?: boolean;
   /** An unread / in-progress count on the icon's corner (0 hides it). */
   badge?: number;
-  /** The page's own address. A page row is a real link (the owner, E31): a middle-click, a
+  /** The page's own address. A page row is a real link (the owner's call): a middle-click, a
    *  Ctrl/Cmd-click or the browser's "Open in new tab" opens the page in a tab of its own. */
   href: string;
   /** The in-app navigation, run only for a plain left click. */
@@ -186,7 +186,7 @@ export interface NavRailGroupPage {
   onSelect: () => void;
 }
 
-/** One rail row standing for a set of rare pages (E31, the owner's pick: the admin pages), opening a
+/** One rail row standing for a set of rare pages (the owner's pick: the admin pages), opening a
  *  panel of them to its right. The rail fits a 133–150% zoom without a scroll nobody sees: five admin
  *  rows sat below the fold behind a fade alone. The row is "you are here" while any of its pages is.
  *  The panel's pages are real links, as the rail's own rows are (a middle-click opens a tab), so it is a