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/Modal.fullscreen.dom.test.tsx

@@ -4,7 +4,7 @@
 // <body> is painted beneath it, i.e. invisible). Leaving fullscreen changes that container — and a
 // React portal whose container changes is torn down and recreated, so everything the dialog held went
 // with it: a half-filled download form reset to its defaults and focus fell to <body> (a verifier's
-// side finding in deep-review round 4).
+// side finding in a review).
 import { describe, it, expect, afterEach } from "vitest";
 import { render, cleanup, screen, fireEvent, act } from "@testing-library/react";
 import { vi } from "vitest";
@@ -58,7 +58,7 @@ describe("a dialog with an autoFocus child", () => {
     // has already called `.focus()` on a disconnected node — a no-op — and Modal's own fallback
     // then puts the focus on the first focusable in the panel, the header's ✕. That is what the
     // command palette, the download rename dialog and every non-danger confirm shipped with after
-    // deep-review round 4 (found in round 5).
+    // the first fix (found by a later review).
     render(
       <Modal initialFocus="close" title="Search" onClose={() => {}}>
         <input aria-label="query" autoFocus />