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/lib/actionErrors.test.ts

@@ -23,7 +23,7 @@ beforeEach(() => {
 describe("a Plex collection name that is already taken", () => {
   it("is reported once, in the reader's language", async () => {
     // The 409 was a bare English string: the global error dialog showed it in English, and the
-    // editor's own toast said the same thing again, translated (deep-review round 4).
+    // editor's own toast said the same thing again, translated.
     vi.spyOn(globalThis, "fetch").mockResolvedValue(
       new Response(JSON.stringify({ detail: { code: "collection", reason: "name_taken" } }), {
         status: 409,
@@ -78,7 +78,7 @@ describe("notifyActionError", () => {
   it("says a bare-English server reason in the reader's language, not the server's", () => {
     // `reportFailure` leaves 403/404/429 to the caller, so for exactly those statuses whatever
     // English a route hand-wrote went straight into the toast — "No user with that email." in an
-    // otherwise Hungarian UI, while the caller's translated fallback sat unused (round 5).
+    // otherwise Hungarian UI, while the caller's translated fallback sat unused.
     notifyActionError(new HttpError(404, "No user with that email."), "TRANSLATED");
     expect(notifyMock).toHaveBeenCalledWith({
       level: "error",