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_channel_backfill.py

@@ -1,4 +1,4 @@
-"""R14 backend coverage: the channel-detail Normal/Shorts/Live counts (moved off the manager table),
+"""Backend coverage: the channel-detail Normal/Shorts/Live counts (moved off the manager table),
 the update_channel -> immediate-backfill wiring, and the interactive
 "get full history" guard that keeps it from racing the scheduler's cursor or draining the budget."""
 
@@ -54,7 +54,7 @@ def test_channel_detail_returns_normal_shorts_live_counts(db, monkeypatch, overr
 
 @pytest.mark.parametrize("hidden", [True, False])
 def test_channel_detail_surfaces_subscription_hidden_flag(db, monkeypatch, hidden, override_dep):
-    # S6: the detail GET must expose the per-subscription `hidden` flag so the channel page can offer
+    # The detail GET must expose the per-subscription `hidden` flag so the channel page can offer
     # the same Unhide toggle the manager has (the flag was previously omitted from the detail dict).
     ch = Channel(id="UC_hid", title="Hidden?", published_at=_PUB)
     db.add(ch)