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_feed_views.py

@@ -1,4 +1,4 @@
-"""Feed search + watch-state views (E2 S3), DB-lane.
+"""Feed search + watch-state views, DB-lane.
 
 Regression nets for two query rewrites that must stay behaviour-equivalent:
 - PERF-12: the search channel-name branch moved from an ORed `unaccent(channels.title) ILIKE` (which
@@ -178,7 +178,7 @@ _SUMMARY_CASES = [
 
 def test_a_deleted_tags_id_filters_to_nothing_not_to_everything(db, as_user, make_user):
     """A saved view or the feed's filter state keeps a tag id after the tag is deleted. The feed used
-    to drop an unknown id, so that view silently showed the WHOLE feed under its old name (E32 S2).
+    to drop an unknown id, so that view silently showed the WHOLE feed under its old name.
     A filter on a tag that no longer exists matches nothing; the chip says the tag is gone."""
     user, tag_ids = _seed_tagged(db, make_user)
     client = as_user(user)
@@ -196,8 +196,8 @@ def test_a_deleted_tags_id_filters_to_nothing_not_to_everything(db, as_user, mak
 
 def test_another_users_private_tag_answers_like_one_that_does_not_exist(db, as_user, make_user):
     """The category split resolved any tag id, so a foreign PRIVATE tag landed in its real category
-    and the summary's facets differed from a non-existent id's: an oracle for which foreign ids exist
-    (E32 review). Both must now read as "missing"."""
+    and the summary's facets differed from a non-existent id's: an oracle for which foreign ids exist.
+    Both must now read as "missing"."""
     user, tag_ids = _seed_tagged(db, make_user)
     stranger = make_user(email="stranger@example.com")
     private = Tag(user_id=stranger.id, name="Secret", category="language")
@@ -247,7 +247,7 @@ def test_untagged_filter_keeps_only_channels_the_user_has_not_tagged(db, as_user
 def test_feed_years_counts_ignore_the_date_narrowing_but_honour_other_filters(
     db, as_user, make_user
 ):
-    """The year facet (item 8 / E8·S2): `/api/feed/years` returns a per-year video count, newest first,
+    """The year facet: `/api/feed/years` returns a per-year video count, newest first,
     with the publish-date narrowing DROPPED (so switching years stays a drill-down) but every other
     filter still applied (here: a channel pick)."""
     user = make_user(email="years@example.com")