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/mailtext.py

@@ -1,5 +1,5 @@
 """Bilingual text for every outbound mail, and the one resolver that decides which language a
-recipient gets (E9.5 S7).
+recipient gets.
 
 Why a catalog instead of literals in `email.py`: HU/EN parity is a house rule for every user-facing
 string, and a mail is the one user-facing surface the reader CANNOT re-render by switching the UI
@@ -82,8 +82,8 @@ def lang_for_email(db: Session, email: str) -> str:
 # a name, not a translated string, and giving them keys would invite someone to "translate" them.
 MAIL_TEXT: dict[str, dict[str, str]] = {
     # ── How long a link lasts ───────────────────────────────────────────────────────────────
-    # Rendered, not interpolated as a raw hour count: the approval mail's token lives seven days
-    # (E9.5 S6), and "expires in 168 hours" is a unit no reader converts.
+    # Rendered, not interpolated as a raw hour count: the approval mail's token lives seven days,
+    # and "expires in 168 hours" is a unit no reader converts.
     "duration.hours": {"en": "{n} hours", "hu": "{n} óra"},
     "duration.days": {"en": "{n} days", "hu": "{n} nap"},
     # ── Shared closing fragments ────────────────────────────────────────────────────────────────