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/playlistColumns.tsx

@@ -25,7 +25,7 @@ export function playlistColumns({
    * so a view-sort never renumbers the playlist under the reader. */
   indexOf: (v: Video) => number;
   onPlay: (v: Video) => void;
-  /** The row's "…" — the visible door to its menu (E28 C1), where "Remove" now lives. */
+  /** The row's "…" — the visible door to its menu, where "Remove" now lives. */
   more: (v: Video) => ReactNode;
 }): Column<Video>[] {
   const cols: Column<Video>[] = [
@@ -70,7 +70,7 @@ export function playlistColumns({
               <img src={v.thumbnail_url} alt="" className="w-full h-full object-cover" />
             )}
             {/* Narrow, the Length column is gone and its value rides on the picture, as on the
-                media card (E28 S3f3, the owner's pick A). */}
+                media card (the owner's pick A). */}
             <span className="pointer-events-none absolute right-0.5 bottom-0.5 @2xl:hidden">
               {lengthCell(v, t, true)}
             </span>
@@ -136,7 +136,7 @@ export function playlistColumns({
     // Room for the "…" at 44px on touch (`.touch-target`), not just its 28px under a mouse.
     width: "3.25rem",
     nowrap: true,
-    // The X that stood here went into the menu with the Delete key and an undo toast (E28 C1): a
+    // The X that stood here went into the menu with the Delete key and an undo toast: a
     // one-click destructive button on every row, for an action the toast now makes reversible.
     render: more,
   });