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/requirements.lock

@@ -2,9 +2,9 @@
 # Source of truth is requirements.txt (the floor ranges); regenerate after editing it with:
 #   docker run --rm -v "$PWD":/b python:3.12-slim sh -c 'pip install -q -r /b/requirements.txt && pip freeze --exclude pip --exclude setuptools --exclude wheel' | sort
 # The prod Dockerfile + Dockerfile.test install THIS file; the gate's pip-audit (scripts/check.sh)
-# scans it, blocking since E32. A security fix may be ONE pin bumped in place instead of a full
+# scans it, blocking. A security fix may be ONE pin bumped in place instead of a full
 # regeneration, which would move every other package too — then `pip install -r requirements.lock && pip check` must
-# pass. In place so far: urllib3 2.7.0 -> 2.8.0 (PYSEC-2026-4175/4176/4177, E32).
+# pass. In place so far: urllib3 2.7.0 -> 2.8.0 (PYSEC-2026-4175/4176/4177).
 APScheduler==3.11.3
 Authlib==1.7.2
 Mako==1.4.1