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_request_models.py

@@ -1,4 +1,4 @@
-"""R6 S1b: the request models turn payloads that used to 500 into clean validation errors,
+"""The request models turn payloads that used to 500 into clean validation errors,
 and PATCH models distinguish an absent field from an explicit null. Pure-logic (no DB) — it
 exercises the Pydantic models the route modules define, not the endpoints."""
 
@@ -51,7 +51,7 @@ def test_admin_quota_rejects_non_numeric_limit():
 
 def test_progress_accepts_fractional_seconds():
     # The player may post a raw currentTime (a float). The field must accept it (truncated in the
-    # handler) — a plain `int` field would 422 on the progress hot path. (code-review round 1.)
+    # handler) — a plain `int` field would 422 on the progress hot path.
     assert VideoProgressIn(position_seconds=12.7).position_seconds == 12.7
     assert int(VideoProgressIn(position_seconds=12.7).position_seconds) == 12
     with pytest.raises(ValidationError):