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_empty_listing_guards.py

@@ -1,11 +1,11 @@
-"""An empty answer is not an emptied library (E32 S2).
+"""An empty answer is not an emptied library.
 
 Every mirror here reconciles: what the remote no longer lists is removed locally. The catalog
 (`_prune_stale`) and the subscriptions (`should_prune`) had long refused to do that on an empty
 listing that contradicts what we hold — a 200 with no items is a blip far more often than a user
 who removed everything. Two mirrors did not: the Plex collections (a row carries the user-set
 `editable` flag, and every member its keys) and the YouTube playlist mirrors. These hold both — and,
-one level down (E32 review), a collection's members and a playlist's items: an empty answer that
+one level down, a collection's members and a playlist's items: an empty answer that
 contradicts the remote's own count is not believed either.
 """
 
@@ -174,7 +174,7 @@ class _YTOneGone(_YT):
 
 def test_a_list_deleted_mid_pull_is_skipped_and_the_rest_still_sync(db, make_user, monkeypatch):
     # Not "this playlist no longer exists" for a bulk pull the user never named a list in, and not
-    # a failure for every other list either (E32 review round 2): the next pull drops the mirror.
+    # a failure for every other list either: the next pull drops the mirror.
     user = make_user()
     db.commit()
     listing = [