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/ContextMenu.dom.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 //
-// Contract of the right-click / long-press context menu (E10 S2). Same floating-ui recipe as
+// Contract of the right-click / long-press context menu. Same floating-ui recipe as
 // ActionMenu.dom.test.tsx: stub the two observers autoUpdate wires and wrap in a FloatingTree, and
 // query with `{ hidden: true }` because floating-ui never positions in jsdom, so the surface stays
 // `visibility: hidden` and the a11y tree treats its contents as hidden.
@@ -176,7 +176,7 @@ describe("useContextMenu / ContextMenu", () => {
 
   // A card the keyboard is on is focused as a whole, through a tabindex=-1 root (lib/listNav), and
   // holds tabbable controls. floating-ui handed the focus back to the first of them instead of the
-  // card — measured in Chrome (deep-review E11 round 2): j, Shift+F10, Escape left it on the thumbnail.
+  // card — measured in Chrome: j, Shift+F10, Escape left it on the thumbnail.
   // Closed here by picking the row (the other way out; jsdom delivers no Escape to a keyboard-opened
   // menu — Escape is measured in the browser).
   it("gives the focus back to a card focused as a whole, not to a control inside it", async () => {