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/Playlists.dom.test.tsx

@@ -392,7 +392,7 @@ describe("Playlists — a change the server refused is taken back on screen", ()
     // refetch that follows a successful reorder did. So a second reorder rejected while that refetch
     // was still in flight rolled the rows back past the FIRST one, which the server had accepted.
     // The membership guard cannot correct an order (its key is the sorted id set), so the wrong
-    // order stayed on screen for good and the next drag persisted it (deep-review round 5).
+    // order stayed on screen for good and the next drag persisted it.
     const reorder = vi
       .spyOn(api, "reorderPlaylist")
       .mockResolvedValueOnce(undefined as never)
@@ -429,7 +429,7 @@ describe("Playlists — a change the server refused is taken back on screen", ()
   });
 });
 
-// E28 C1: a row's X column went. Remove is in the row's menu and on the Delete key, with no confirm —
+// A row's X column went. Remove is in the row's menu and on the Delete key, with no confirm —
 // the toast's Undo puts the row back where it stood, in ONE request.
 describe("Playlists — removing a row, and undoing it", () => {
   // The server's side of a removal, so the refetch after it agrees with it.
@@ -489,7 +489,7 @@ describe("Playlists — removing a row, and undoing it", () => {
     await waitFor(() => expect(api.addToPlaylist).toHaveBeenCalledWith(1, "A", 0));
   });
 
-  // E28 review C4: the stored index drifts when the list changes before the Undo (a row added at
+  // The stored index drifts when the list changes before the Undo (a row added at
   // the top in another tab) — the row goes back after its old neighbour instead.
   it("puts it back after its neighbour when the list changed before the Undo", async () => {
     const lists = { 1: [vid("A", 0), vid("B", 0), vid("C", 0)] };
@@ -566,7 +566,7 @@ describe("Playlists — removing a row, and undoing it", () => {
   });
 });
 
-// E28 C1: the header keeps Play all; rename, YouTube and delete are the list's menu, behind a "…".
+// The header keeps Play all; rename, YouTube and delete are the list's menu, behind a "…".
 describe("Playlists · the header's '…'", () => {
   it("replaces the pencil, the YouTube buttons and the trash, and renames from its menu", async () => {
     mount({ 1: [vid("A", 0)] });
@@ -615,7 +615,7 @@ describe("Playlists · a narrow table", () => {
       el.classList.contains("hidden") && el.classList.contains("@2xl:table-cell");
     const heads = [...document.querySelectorAll("thead th")];
     const cells = [...document.querySelectorAll("tbody tr:first-child td")];
-    // Columns: grip, #, title, channel, length, "…". Length went too (E28 S3f3, the owner's pick A).
+    // Columns: grip, #, title, channel, length, "…". Length went too (the owner's pick).
     expect(heads.map(narrowHidden)).toEqual([false, true, false, true, true, false]);
     expect(cells.map(narrowHidden)).toEqual([false, true, false, true, true, false]);
   });
@@ -661,7 +661,7 @@ describe("Playlists — the rows are a keyboard list", () => {
 });
 
 // The header's meta line and its view-controls gate were typechecked and linted but never rendered,
-// so a regression in either shipped green (E9 review). The count and the runtime sum the WHOLE
+// so a regression in either shipped green. The count and the runtime sum the WHOLE
 // list; an unknown duration counts as nothing rather than poisoning the sum; and a list of one has
 // no view to change.
 describe("Playlists · the header", () => {