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_logs.py

@@ -1,4 +1,4 @@
-"""E22 S2 — the one logging configuration, its redaction, and the request line that replaced
+"""The one logging configuration, its redaction, and the request line that replaced
 uvicorn's access log.
 
 Redaction is the part worth guarding hardest: it is applied at the HANDLER, so these cases are the
@@ -148,7 +148,7 @@ def test_every_line_carries_the_agreed_fields(sink):
 
 
 def test_a_plain_log_call_still_produces_a_well_formed_line(sink):
-    """An event code is optional — 125 existing call sites have none yet (E22.5 adds them), and they
+    """An event code is optional — 125 existing call sites have none yet, and they
     must not be worse off than before this module."""
     logging.getLogger("siftlode.test").warning("just a message")
     payload = sink.objects()[-1]