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/e2e/nav.spec.ts

@@ -6,9 +6,9 @@ import { openApp } from "./helpers";
 // this exercises is precisely what the module-registry refactor replaces.
 test.describe("page switching", () => {
   // Core + admin modules (the E2E account is an admin, matching the real primary user), so this
-  // also exercises the admin pages. Since E31 those sit behind ONE rail row ("Administration",
+  // also exercises the admin pages. They sit behind ONE rail row ("Administration",
   // `nav-admin`) that opens a panel of them — the spec still listed them as rail rows of their own,
-  // and it failed on every run from E31 to E32 without anyone seeing it, because e2e was in no
+  // and it failed on every run for a while without anyone seeing it, because e2e was in no
   // required gate (it is now part of `release`).
   const RAIL = ["channels", "playlists", "stats", "settings", "feed"];
   const ADMIN = ["scheduler", "config", "users", "audit"];