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_email_canonical.py

@@ -1,4 +1,4 @@
-"""One mailbox, many spellings — the canonical-email identity rule (E9.5 S3).
+"""One mailbox, many spellings — the canonical-email identity rule.
 
 Gmail delivers `x.y@gmail.com`, `xy@gmail.com` and `xy+tag@gmail.com` to ONE inbox. The invite gate
 used to compare literal strings, so on 2026-09-03 an approved user's Google sign-in became a silent
@@ -351,9 +351,9 @@ def test_me_reports_the_google_address_only_when_it_differs(client, db, make_use
 
 
 def test_rerunning_the_wizard_with_another_gmail_spelling_updates_the_same_admin(db):
-    """The wizard can be re-run, and E9.5's premise is that two spellings of one Gmail mailbox are one
+    """The wizard can be re-run, and the premise is that two spellings of one Gmail mailbox are one
     person. Resolving by the literal string here would create a SECOND admin row — exactly the
-    duplicate the epic exists to prevent. This site was outside S3's sweep (DX-17).
+    duplicate canonical email exists to prevent. This site was missed by the first sweep (DX-17).
 
     Calls the handler directly: `require_setup` is an instance-state gate, not the behaviour under
     test, and the test fixtures present an already-configured instance."""