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_detail.py

@@ -1,4 +1,4 @@
-"""Regression (E5 S5b): the info-page rich detail (IMDb score/id, tagline, full cast) is persisted at
+"""Regression: the info-page rich detail (IMDb score/id, tagline, full cast) is persisted at
 rich-fetch time and served from the DB row instead of a live plex.metadata() call.
 
 - `rich_detail`/`apply_rich_detail` are pure — parse + persist the rich-only fields.
@@ -64,7 +64,7 @@ def test_rich_detail_prefers_imdb_entry_and_builds_link():
 
 
 def test_rich_detail_is_imdb_only_no_audience_fallback():
-    # E5 S6: imdb_rating is IMDb-ONLY. With no imdb:// Rating entry it's NULL — the Plex audience score
+    # Imdb_rating is IMDb-ONLY. With no imdb:// Rating entry it's NULL — the Plex audience score
     # is NOT mislabelled as an IMDb score (it's served separately via the `rating` column).
     d = detail.rich_detail({"audienceRating": 7.25})
     assert d["imdb_rating"] is None
@@ -113,7 +113,7 @@ class _StubClient:
 def test_apply_facet_fields_preserves_enriched_cast_names():
     """A full sync must NOT downgrade an already-enriched full cast back to the listing top-20 — since
     sync never clears people_enriched_at, nothing would regrow it and the person-page filter would lose
-    the names past the listing cap (regression guard for E5 S5b's enrich gating)."""
+    the names past the listing cap (regression guard for the enrich gating)."""
     # The cheap listing's top few cast + the (always-present) director.
     thin = {
         "Role": [{"tag": "Al Pacino"}, {"tag": "James Caan"}],
@@ -357,7 +357,7 @@ def test_show_detail_lazy_fills_when_unenriched(db, monkeypatch, override_dep):
 def test_item_detail_names_tracks_from_their_language_not_plexs_title(
     db, monkeypatch, override_dep
 ):
-    """E30: Plex titles a track in the Plex SERVER's language ("English (AC3 5.1)", "magyar Forced",
+    """Plex titles a track in the Plex SERVER's language ("English (AC3 5.1)", "magyar Forced",
     "中文 (EAC3 Stereo)"), so a Hungarian reader got a mix. The route sends what the client names it
     from instead — the language tag, the forced / SDH flags, the codec tail — keeping Plex's title
     only as the fallback, and no invented English "Audio 2" for a track with neither."""