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/email.py

@@ -197,7 +197,7 @@ def _next_step(verify_link: str | None, has_account: bool, hours: int, lang: str
 
     Three states, because a mail that guesses wrong sends someone looking for a link that isn't
     coming — or worse, tells them to sign in when their address is still unconfirmed and sign-in will
-    refuse them (E9.5 S4 named that state on screen; the mail must not contradict it).
+    refuse them (the sign-in screen names that state; the mail must not contradict it).
 
     The link branch is the point of the C option: in the common ordering — approve first, the person
     signs up later, or signs up and waits — they would otherwise have to find and open TWO separate
@@ -253,7 +253,7 @@ def send_access_invitation(
     the closing line offers no deadline — nudging is the one thing this mail must not do.
 
     Its language is the one place the app has no signal to read: the admin typed this address in, so
-    nobody has ever told us how to address its owner. That is why the send button asks (E9.5 S7).
+    nobody has ever told us how to address its owner. That is why the send button asks.
     """
     admin, contact = _reply_contact(lang)
     body = mailtext.text(