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_hls_reaper.py

@@ -2,15 +2,15 @@
 
 plex/stream.py promises "a periodic reaper kills idle ones", but `reap_idle` ran only inside
 POST /session: after the last playback, an abandoned session (its ffmpeg still working through the
-film, its directory a whole segmented copy of it) lived until somebody pressed play again
-(deep-review round 4).
+film, its directory a whole segmented copy of it) lived until somebody pressed play again.
 
 And it must not take a PAUSED viewer's session: a remux finishes long before the film does, and a
 paused player with a full buffer fetches nothing, so "finished and unfetched for 30 s" does not mean
-"abandoned". Round 4 bought that safety by keeping EVERY finished session for the full ten-minute
-idle window — one segmented copy of a film per item, on the volume the download center shares. Round
-5 replaced the blanket retention with the signal that already existed: an open player checkpoints its
-position every 10 s whether or not it is playing, and that route says "still here".
+"abandoned". An earlier fix bought that safety by keeping EVERY finished session for the full
+ten-minute idle window — one segmented copy of a film per item, on the volume the download center
+shares. Its successor replaced the blanket retention with the signal that already existed: a player
+parked mid-item checkpoints its position every 10 s, paused or not, and that route says "still
+here" (it stops once the item is finished, where reaping is correct).
 
 Both call sites are pinned here, because the bug each time was that only one of them moved: the
 periodic pass, and the fact that starting a session no longer sweeps OTHER viewers' sessions at all.
@@ -174,7 +174,7 @@ def test_starting_a_session_leaves_another_viewers_paused_one_alone(
 ):
     """The opportunistic sweep this route used to run was not scoped to the key being started: it
     walked the WHOLE registry with the 30 s finished rule, so pressing play on any item killed a
-    paused viewer's session — the very case round 4 set out to protect (found in round 5)."""
+    paused viewer's session — the very case the retention fix set out to protect."""
     monkeypatch.setattr(state, "_configured_cache", True)
     monkeypatch.setattr(plex_paths, "local_media_path", lambda db_, fp: "/local/a.mkv")
     started = _library_item(db, "starting")