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/ConfigPanel.tsx

@@ -159,7 +159,7 @@ export default function ConfigPanel() {
       notifyActionError(e, t("config.plexTestFailed"));
     },
   });
-  // Bug reports' "Test connection" (E16): the repository answers, takes issues, is private.
+  // Bug reports' "Test connection": the repository answers, takes issues, is private.
   const testIssues = useMutation({
     mutationFn: () => api.testIssues(),
     onSuccess: (r) =>
@@ -192,7 +192,7 @@ export default function ConfigPanel() {
     return <LoadingState label={t("config.loading")} />;
   }
 
-  // The groups in the registry's sections, in order (E31: sixteen tabs wrapped to three rows at 133%
+  // The groups in the registry's sections, in order (sixteen tabs wrapped to three rows at 133%
   // zoom, so they are a side list under four headings, as Settings has).
   const groupKeys = data.sections.flatMap((s) => s.groups);
   // Clamp the persisted tab to a real group (the registry can change between sessions). The
@@ -214,7 +214,7 @@ export default function ConfigPanel() {
       <div className="px-4 pb-24 pt-3 max-w-4xl w-full mx-auto">
         <p className="text-xs text-muted mb-3 leading-relaxed">{t("config.intro")}</p>
         {/* The groups as a side list under their headings, the fields beside it: Settings' own
-            layout, one pattern for both settings pages (E31). */}
+            layout, one pattern for both settings pages. */}
         <div className={`${SURFACE.panel} mb-4 flex flex-col overflow-hidden sm:flex-row`}>
           <Tabs
             idBase={panel.idBase}