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.

backend/tests/test_youtube_refusals.py

@@ -1,5 +1,5 @@
 """An action YouTube refuses answers a 422 the UI can translate — never a raw 500 — and says what the
-user can do about it (`routes/_common.youtube_refusal`). E9 F1: the two manual syncs had no handler
+user can do about it (`routes/_common.youtube_refusal`). The two manual syncs had no handler
 at all, so a quota or transport failure opened the blocking "Server error" dialog."""
 
 from types import SimpleNamespace
@@ -41,7 +41,7 @@ def test_a_failure_is_sorted_by_what_the_user_can_do(exc, reason):
 def test_a_missing_playlist_is_gone_only_on_a_route_about_that_playlist():
     # A playlist deleted on YouTube: neither a retry nor a reconnect helps (it said "reconnect").
     # Said only where the user picked that one list — anywhere else "this playlist no longer
-    # exists" names nothing on their screen (E32 review round 2).
+    # exists" names nothing on their screen.
     exc = YouTubeError("m", reason="playlistNotFound", path="playlists")
     assert refusal_reason(exc, playlist=True) == "gone"
     assert refusal_reason(exc) == "unavailable"
@@ -71,7 +71,7 @@ class _NoUploads:
 
 def test_a_channel_without_uploads_backfills_as_done_and_is_not_charged_again(db):
     # The recent pass stamped nothing on this answer, so the channel was picked — and charged a
-    # quota unit, with an ERROR traceback — on every backfill run, forever (E32 review round 3).
+    # quota unit, with an ERROR traceback — on every backfill run, forever.
     ch = Channel(id="UCempty", title="Empty", uploads_playlist_id="UUempty")
     db.add(ch)
     db.commit()