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/ui/Combobox.dom.test.tsx

@@ -207,7 +207,7 @@ describe("Combobox", () => {
   });
 
   it("names a chip by a label that arrives with later options, in that same render", () => {
-    // E32 S3: the label cache was a ref an effect filled AFTER the render that needed it, and nothing
+    // The label cache was a ref an effect filled AFTER the render that needed it, and nothing
     // re-rendered afterwards — the chip kept showing the raw id until something unrelated did.
     const view = (options: { value: string; label?: string }[]) => (
       <FloatingTree>
@@ -298,12 +298,12 @@ describe("Combobox", () => {
   });
 });
 
-// PageUp/PageDown (E11, owner request): the sidebar's channel filter and the Plex pickers run to
+// PageUp/PageDown (owner request): the sidebar's channel filter and the Plex pickers run to
 // hundreds of rows, and floating-ui's list navigation has only the arrows. jsdom has no layout, so a
 // page is the fixed fallback of 10 rows; the measured page is pinned in lib/listPaging.
-// E28 review J1 + J2: a failed list said "No matches" (an error dressed as an empty result), and a
+// A failed list said "No matches" (an error dressed as an empty result), and a
 // picker whose pick was still being written took another pick.
-// E28 review F5: a chip's × pressed from the keyboard unmounted with the focus on it — to <body>,
+// A chip's × pressed from the keyboard unmounted with the focus on it — to <body>,
 // out of the picker. The focus goes to the next chip's × (else the previous one, else the field).
 describe("Combobox · removing a chip from the keyboard", () => {
   function Harness({ initial }: { initial: string[] }) {
@@ -335,7 +335,7 @@ describe("Combobox · honest states", () => {
     expect(screen.queryByText("common.noResults")).toBeNull();
   });
 
-  // A failed list with no way back but closing its dialog was a dead end (E28 review, owner go).
+  // A failed list with no way back but closing its dialog was a dead end.
   it("offers a Retry on the error, by click and by Enter in the field, keeping the focus", () => {
     const onRetry = vi.fn();
     wrap(
@@ -363,7 +363,7 @@ describe("Combobox · honest states", () => {
     expect(document.activeElement).toBe(field);
   });
 
-  // Round 2 (F4): a listbox may own options only — the alert, "Loading…", "No matches" and the
+  // A listbox may own options only — the alert, "Loading…", "No matches" and the
   // "n more" line sat inside it (axe: aria-required-children). They sit beside it now.
   it("keeps the lines about the list outside the listbox, which the field still controls", () => {
     const { rerender } = wrap(