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/app/routes/setup.py

@@ -71,10 +71,10 @@ def setup_admin(body: SetupAdminIn, db: Session = Depends(require_setup)) -> dic
         raise refused(400, "invalid_email")
     if len(password) < PASSWORD_MIN_LEN:
         raise refused(400, "weak_password", min=PASSWORD_MIN_LEN)
-    # Resolve canonically, not by the literal string (DX-17). The wizard can be re-run, and E9.5's
-    # whole point is that two spellings of one Gmail mailbox are one person — matching literally here
-    # would let a second run under the other spelling create the DUPLICATE admin row the epic exists
-    # to prevent. Every other lookup went canonical in S3; this one was outside that diff.
+    # Resolve canonically, not by the literal string (DX-17). The wizard can be re-run, and the
+    # whole point of canonical email is that two spellings of one Gmail mailbox are one person —
+    # matching literally here would let a second run under the other spelling create the DUPLICATE
+    # admin row it exists to prevent. Every other lookup was canonical already; this one was missed.
     user = db.execute(
         select(User).where(User.email_canonical == canonical_email(email))
     ).scalar_one_or_none()