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_overlay.py

@@ -1,4 +1,4 @@
-"""DB-backed regression (E5 S5a): the HU-overlay sync writes Hungarian metadata onto the matched EN
+"""DB-backed regression: the HU-overlay sync writes Hungarian metadata onto the matched EN
 rows (paired by guid), never creating browsable HU rows, and overlay libraries are excluded from the
 browse scope. Also covers the canonical-guid parse and that the generated search_vector_hu populates.
 
@@ -197,7 +197,7 @@ def test_overlay_reconcile_clears_stale_hu_when_twin_vanishes(db):
 
 
 def test_a_listing_cut_short_keeps_the_hungarian_text_it_did_not_reach(db):
-    """E32 S2: the reconcile clears HU text only after a COMPLETE listing. A walk a Plex hiccup cut
+    """The reconcile clears HU text only after a COMPLETE listing. A walk a Plex hiccup cut
     short (an empty page before the server's total), or an empty section, says nothing about the
     titles it did not reach — clearing them dropped their Hungarian text until the next full sync."""
     en_lib = PlexLibrary(plex_key="4", title="Movies", kind="movie")
@@ -304,7 +304,7 @@ class _KindStub:
 
 
 def test_a_show_overlay_writes_hungarian_episode_titles_and_summaries(db):
-    """E30: the HU TV section lists every episode in Hungarian; they were never read, so a Hungarian
+    """The HU TV section lists every episode in Hungarian; they were never read, so a Hungarian
     reader saw every episode title and summary in English."""
     en_lib = PlexLibrary(plex_key="1", title="TV Shows", kind="show")
     hu_lib = PlexLibrary(plex_key="9", title="Sorozatok", kind="show", metadata_lang="hu")
@@ -341,7 +341,7 @@ def test_a_show_overlay_writes_hungarian_episode_titles_and_summaries(db):
 
 
 def test_the_overlay_gives_every_row_one_hungarian_word_per_genre(db):
-    """E30: Plex's HU agent mixes "Action" with "Dráma", and a title with no HU twin kept only its
+    """Plex's HU agent mixes "Action" with "Dráma", and a title with no HU twin kept only its
     English list — so the HU genre list offered "Drama" beside "Dráma"."""
     en_lib = PlexLibrary(plex_key="4", title="Movies", kind="movie")
     hu_lib = PlexLibrary(plex_key="9", title="Filmek", kind="movie", metadata_lang="hu")
@@ -407,7 +407,7 @@ def test_a_twin_without_genres_keeps_the_rows_own_hungarian_list(db):
 
 
 def test_every_synced_row_gets_hungarian_genres():
-    """E30 review: not only an overlaid row — the HU facets and filter read genres_hu for all."""
+    """Not only an overlaid row — the HU facets and filter read genres_hu for all."""
     from app.plex.sync import _apply_facet_fields
 
     row = PlexItem(title="Solo")