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_facets_grid_parity.py

@@ -1,6 +1,6 @@
-"""E3 S1 characterization net — /library grid ⟷ /facets equivalence across watch-state scopes.
+"""Characterization net — /library grid ⟷ /facets equivalence across watch-state scopes.
 
-The Plex god-module split (E3 S1) extracts the movie/show watch-state predicates that are currently
+The Plex god-module split extracts the movie/show watch-state predicates that are currently
 HAND-DUPLICATED in two places: `unified_library` (the grid) and `facets.wsq` (the facet aggregation).
 The facets docstring promises the facets "reflect exactly the visible result set" — i.e. with NO
 metadata filter active (only the `show` state scope), a facet's genre/content-rating counts must equal
@@ -218,7 +218,7 @@ def test_hidden_movie_absent_from_grid_and_facets(db, as_user):
 
 def test_a_watch_state_scope_rolls_the_episodes_up_once(db, as_user):
     """The per-user episode rollup behind a show's watch state is a pass over every episode; each show
-    facet group asked it again (E31 S4e: half of a filtered facets call). It is asked once now."""
+    facet group asked it again (half of a filtered facets call). It is asked once now."""
     from sqlalchemy import event
 
     from app.db import engine