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/auditColumns.tsx

@@ -34,7 +34,7 @@ function counts(after: Record<string, unknown>): JobResult | null {
   return Object.keys(out).length ? out : null;
 }
 
-/** The details of the rows whose server `summary` is a fixed English line (E30), rebuilt from the row's
+/** The details of the rows whose server `summary` is a fixed English line, rebuilt from the row's
  *  data in the reader's language: a job run names the job and its result (as the inbox does), a config
  *  change names the field and its values, an interval or batch change its new value, a count-bearing
  *  action its counts; a pause/resume says nothing the action has not. `undefined` = not one of these (or
@@ -146,7 +146,7 @@ export function auditColumns(
         const local = localDetails(t, r, lang);
         const summary = local ? local.head : r.summary;
         const change = local ? local.sub : changeText(r);
-        // A failed run is the row to see in a page of routine ones (E31): its line in the danger colour.
+        // A failed run is the row to see in a page of routine ones: its line in the danger colour.
         const failed = r.action === "job_run" && r.after?.status === "error";
         return (
           // wrap-break-word (wrap), NOT truncate: in the auto-layout table a `truncate` (nowrap) cell