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/PlexSidebar.tsx

@@ -69,7 +69,7 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
   const facets = facetsQ.data;
 
   const fCount = plexFilterCount(filters);
-  // Scope and sort moved to the header / above-the-grid toolbar (S3); the panel badge now counts only
+  // Scope and sort moved to the header / above-the-grid toolbar; the panel badge now counts only
   // the filters that still live in this panel (expanded filters + watch-state).
   const activeCount = fCount + (show !== "all" ? 1 : 0);
 
@@ -345,7 +345,7 @@ export function PlaylistPicker() {
   const options: ComboboxOption[] = [
     ...matches.map((p) => ({ value: `pl:${p.id}`, label: p.title, count: p.item_count })),
     // Offer to create only when the typed name isn't already a playlist — which is unknown until the
-    // lists load: over a failed load the row pushed out the error and its Retry (round 2, P7).
+    // lists load: over a failed load the row pushed out the error and its Retry.
     ...(q && !exact && playlistsQ.data ? [{ value: CREATE_PLAYLIST, label: q }] : []),
   ];
 
@@ -355,7 +355,7 @@ export function PlaylistPicker() {
       qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
       onOpenPlaylist(pl.id);
     } catch (e) {
-      // Called without an await from the picker, so a refusal was an unhandled rejection (E11 S2b).
+      // Called without an await from the picker, so a refusal was an unhandled rejection.
       notifyActionError(e, t("plex.playlistAdd.createFailed"));
     }
   }
@@ -403,7 +403,7 @@ export function PlaylistPicker() {
   );
 }
 
-// The collection filter (E5 S7 → combobox; now multi-select): a combi-input matching the genre/studio
+// The collection filter (a multi-select combobox): a combi-input matching the genre/studio
 // pickers, its vocabulary searched SERVER-side (the union can run to hundreds). The generic <Combobox>
 // owns the field/keyboard/ARIA + the any/all match footer; this wrapper owns the query→fetch (gated on
 // `open`) and maps collections to options. A title can belong to several collections, so multiple can