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_plex_lang.py

@@ -1,4 +1,4 @@
-"""E5 S6: HU-overlay language serving. When ?lang=hu, the unified library, facets, genre filter and
+"""HU-overlay language serving. When ?lang=hu, the unified library, facets, genre filter and
 detail endpoints serve the Hungarian `*_hu` columns (falling back to EN where a row has no HU twin);
 ?lang=en (the default) serves English. Also covers the imdb_rating IMDb-only / audience-score split.
 DB lane (`db` fixture) — skipped without Postgres.
@@ -119,7 +119,7 @@ def test_genre_filter_matches_active_language_vocabulary(db, monkeypatch, overri
 
 
 def test_a_genre_filter_saved_in_english_still_matches_in_hungarian(db, monkeypatch, override_dep):
-    """E30: the HU column holds one Hungarian word per genre. A filter saved while the HU list still
+    """The HU column holds one Hungarian word per genre. A filter saved while the HU list still
     offered "Action" is read through the same map, so it finds the "Akció" title instead of nothing."""
     user = _seed(db)
     resp = _get(
@@ -129,7 +129,7 @@ def test_a_genre_filter_saved_in_english_still_matches_in_hungarian(db, monkeypa
 
 
 def test_a_genre_the_hungarian_list_offers_in_english_still_matches(db, monkeypatch, override_dep):
-    """E30 review: a row the sync has not given Hungarian genres yet is listed under its English word
+    """A row the sync has not given Hungarian genres yet is listed under its English word
     (the facets read coalesce(genres_hu, genres)); filtering by that word must find it."""
     user = _seed(db)
     facets = _get(db, user, monkeypatch, override_dep, "/api/plex/facets?scope=movie&lang=hu")
@@ -161,7 +161,7 @@ def test_item_detail_language_and_rating_split(db, monkeypatch, override_dep):
     body = resp.json()
     assert body["title"] == "Magyar Cím"
     assert body["summary"] == "Magyar összefoglaló."
-    assert body["tagline"] == "Magyar mottó."  # E30: the HU overlay's tagline
+    assert body["tagline"] == "Magyar mottó."  # the HU overlay's tagline
     en = _get(db, user, monkeypatch, override_dep, "/api/plex/item/m1").json()
     assert en["tagline"] == "English tagline."
     # imdb_rating is IMDb-only (None here); the audience score is served separately as `rating`.
@@ -285,7 +285,7 @@ def _seed_rails(db):
 
 
 def test_every_card_rail_serves_the_readers_language(db, monkeypatch, override_dep):
-    """E30: the grid spoke Hungarian, but every other card surface (Discover rails, show page,
+    """The grid spoke Hungarian, but every other card surface (Discover rails, show page,
     search, collection, person page, playlist, Home) served the English title to a HU reader."""
     user, pid = _seed_rails(db)