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_playlist_sync_claim.py

@@ -1,4 +1,4 @@
-"""E28 review C1 — one YouTube sync of a playlist at a time, whoever starts it.
+"""One YouTube sync of a playlist at a time, whoever starts it.
 
 A push reads YouTube and then writes both sides for seconds, committing step by step, so the
 client's in-flight flag (one page's menus) and a row lock (gone at the first commit) both leave two
@@ -67,7 +67,7 @@ def test_push_and_revert_refuse_while_the_list_is_claimed(
             r = c.post(f"/api/playlists/{pl.id}/{path}")
             assert r.status_code == 409
             assert r.json()["detail"] == {"code": "playlist", "reason": "sync_busy"}
-        # a delete too: a push mid-way would plan the deleted list as emptied (E28 round 4)
+        # a delete too: a push mid-way would plan the deleted list as emptied
         r = c.delete(f"/api/playlists/{pl.id}")
         assert r.status_code == 409
         assert r.json()["detail"] == {"code": "playlist", "reason": "sync_busy"}
@@ -132,7 +132,7 @@ def test_the_scheduled_refresh_skips_a_mirror_being_synced(db, make_user, monkey
 def test_refused_reverts_do_not_spend_the_pull_cooldown(
     db, as_user, make_user, override_dep, monkeypatch
 ):
-    """Round 2: the cooldown (three pulls per ten minutes) is checked after the claim, so clicking
+    """The cooldown (three pulls per ten minutes) is checked after the claim, so clicking
     Revert while a push runs is refused as busy without using up a pull."""
     user = make_user()
     pl = _linked(db, user)