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/app/routes/plex/discover.py

@@ -1,11 +1,11 @@
-"""Plex Discover: the rail-surface read-model (E5 S4).
+"""Plex Discover: the rail-surface read-model.
 
 A consolidated payload for the dedicated Discover landing — a stack of horizontal rails:
 Continue watching · Recently added · Browse-by-genre tiles · Because you watched X — plus a
 separate, reshuffling Surprise-me sample. Serve-from-DB only: the recommendation rail reads the
-recs the S2 TMDB engine already persisted (`PlexItem.recommendations`), never a live TMDB call, and
+recs the TMDB engine already persisted (`PlexItem.recommendations`), never a live TMDB call, and
 every card comes from the same `movie_card`/`show_card` serializers the grid uses (so rails inherit
-S1's `art` + S3's rating/summary for free). The rails carry EN metadata (like the other non-grid
+their `art` + rating/summary for free). The rails carry EN metadata (like the other non-grid
 card paths); the HU overlay coalesces only in the main grid.
 
 The sidebar metadata filters (genre / rating / year / collection / …) narrow EVERY rail too, so the
@@ -114,8 +114,8 @@ def _recently_added(
         else []
     )
     # Merge the two type-homogeneous lists by added_at, then serialize each side in one batch so the
-    # per-card state lookups stay batched (no N+1). Same `or NO_TS` key as the two rails below — the
-    # E24 review found this one still on the older `(is not None, value)` form, i.e. one rule in two
+    # per-card state lookups stay batched (no N+1). Same `or NO_TS` key as the two rails below — a
+    # review found this one still on the older `(is not None, value)` form, i.e. one rule in two
     # spellings, with the two comments explaining it differently.
     tagged = [(m.added_at, "movie", m) for m in movies] + [(s.added_at, "show", s) for s in shows]
     tagged.sort(key=lambda t: t[0] or rollup.NO_TS, reverse=True)