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_admin_pending_access.py

@@ -1,4 +1,4 @@
-"""The admin lifeline for a sign-up that is stuck outside (E9.5 S5, roadmap UX-25), DB-lane.
+"""The admin lifeline for a sign-up that is stuck outside, DB-lane.
 
 Approve an address BEFORE the person registers and their sign-up produces no queue row at all —
 `_decide` only takes a still-pending invite — so the Access-requests screen stayed empty while they
@@ -133,7 +133,7 @@ def test_an_unconfirmed_account_is_listed_and_can_be_sent_a_fresh_link(
         lambda to, lang, link, **kw: sent.append((to, link)) is None,
     )
     admin = make_user(email="admin@example.com", role="admin")
-    # Active but unconfirmed — a real state since E9.5 S1 made the two gates order-independent. It is
+    # Active but unconfirmed — a real state now that the two gates are order-independent. It is
     # not the admin's to decide, but the mail may simply never have arrived, so it is theirs to help
     # with. The approved invite is what puts them on this screen at all: the list is "the address may
     # sign in, this account still can't", so an undecided request stays with the queue that owns it.