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/SettingsPanel.tsx

@@ -146,7 +146,7 @@ function Appearance({ prefs }: { prefs: PrefsController }) {
   const { t } = useTranslation();
   // Controlled by PrefsProvider: each change applies locally at once and auto-saves (debounced).
   const { theme, setTheme, perf, setPerf, hints, setHints, railLabels, setRailLabels } = prefs;
-  // The landing page (E24). Per-account localStorage, not a server preference like the rest of this
+  // The landing page. Per-account localStorage, not a server preference like the rest of this
   // tab: `loadInitialPage` reads it synchronously in NavigationProvider's initializer. The two stored
   // values come from navigationInit, which is what reads them back at boot.
   const [startOnHome, setStartOnHome] = useAccountPersistedState(LS.startOnHome, START_ON_HOME.on);
@@ -611,7 +611,7 @@ function PlexWatchSync() {
   );
 }
 
-// Per-user download authentication (S6): store your own login cookies for an auth-walled host
+// Per-user download authentication: store your own login cookies for an auth-walled host
 // (e.g. a private Facebook group you belong to) so downloads from it use YOUR login. Write-only —
 // the stored blob is never shown back; each row shows only the host + whether it's set + when.
 // A cookie-authed download is kept private to your account (never cache-shared with another user).