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/QueryState.tsx

@@ -73,7 +73,7 @@ export function LoadingState({
 }
 
 // Centered inline status line for a page/section's loading and error(+retry) states, in the house
-// style (mirrors Playlists' error block). Text-only by design — R3's goal is to stop pages
+// style (mirrors Playlists' error block). Text-only by design — the goal is to stop pages
 // rendering an error or a still-loading fetch AS an "empty" state, not to add skeletons. The
 // `role=status`/`aria-live` also gives screen readers the state change (the app had zero live
 // regions before).
@@ -118,6 +118,6 @@ export function StateMessage({
 
 // The honest-state convention every page now follows: check `isError && <no data>` BEFORE the empty
 // branch, so a failed fetch renders this message (with Retry) instead of the page's "empty" text
-// — the R3 bug where a backend restart read as "No channels". The `&& <no data>` guard matters: a
+// — the bug where a backend restart read as "No channels". The `&& <no data>` guard matters: a
 // transient refetch error while data is already on screen keeps the data rather than blanking it to
 // an error. `StateMessage` is the shared building block pages wire into their loading/empty ternary.