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/Scheduler.dom.test.tsx

@@ -2,7 +2,7 @@
 //
 // The scheduler's two number editors — a job's interval and the maintenance batch size. Both used to
 // hold a MIRROR of the server value that an effect re-synced; both now show the server value and keep
-// a draft only while the field is being edited (E27 S2). That matters here more than elsewhere
+// a draft only while the field is being edited. That matters here more than elsewhere
 // because this page polls: every few seconds a fresh status arrives, and a mirror is exactly the
 // thing a poll can overwrite at the wrong moment.
 import { describe, it, expect, vi, afterEach, beforeEach } from "vitest";
@@ -81,7 +81,7 @@ afterEach(() => {
 const intervalBox = () => screen.queryByRole("spinbutton", { name: "" });
 const batchBox = () => screen.getByLabelText("scheduler.maintenance.batchLabel");
 
-// The FIRST FOUR tests below are CHARACTERIZATION, not regression guards for E27 S2: the old
+// The FIRST FOUR tests below are CHARACTERIZATION, not regression guards for the draft sweep: the old
 // JobRow's re-sync effect was already guarded by `if (!editing)` and the mirror only rendered while
 // editing, so each of those four passes against the pre-sweep code too. They are here because the
 // editor had no tests at all. The two after them — the range refusal and the focus return — ARE
@@ -167,7 +167,7 @@ describe("Scheduler · the job interval editor", () => {
     );
   });
 
-  // E28 audit: a real Enter re-opened the editor — the keystroke's own activation landed on the trigger
+  // A real Enter re-opened the editor — the keystroke's own activation landed on the trigger
   // the focus had just returned to. The closing keys cancel their default action.
   it("closing by a key does not press the trigger again", async () => {
     await mount();
@@ -180,7 +180,7 @@ describe("Scheduler · the job interval editor", () => {
 
 describe("Scheduler · the maintenance batch editor", () => {
   it("keeps what you are typing when a poll changes the batch underneath", async () => {
-    // THE regression this editor needed: its pre-E27 effect re-synced on every
+    // THE regression this editor needed: its old effect re-synced on every
     // `revalidate_batch` change with no editing guard, so a poll landing mid-keystroke replaced
     // what the admin had typed.
     await mount();
@@ -242,7 +242,7 @@ describe("Scheduler · the maintenance batch editor", () => {
   });
 });
 
-describe("Scheduler while paused (E28 audit F13)", () => {
+describe("Scheduler while paused", () => {
   // Unavailable WITH a reason ("resume first"): the runs stay in the tab order, so a keyboard user
   // reaches the hint, and they are inert. Natively disabled, the reason was mouse-only.
   it("keeps Run all and a job's Run focusable and inert", async () => {
@@ -260,7 +260,7 @@ describe("Scheduler while paused (E28 audit F13)", () => {
   });
 });
 
-describe("a job's last result (E30)", () => {
+describe("a job's last result", () => {
   it("is named from its data, not the server's English one-liner", async () => {
     const base = status();
     await mount({