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/main.py

@@ -131,7 +131,7 @@ async def _reap_hls_sessions() -> None:
     A FINISHED session is reaped on the short rule here, not kept for the full idle window: an open
     player checkpoints every 10 s and that route touches the session, so a paused viewer is no longer
     invisible to this pass (see `reap_idle`). Keeping every finished session for ten minutes instead
-    held one whole segmented copy of a film per item on the download volume (round 5)."""
+    held one whole segmented copy of a film per item on the download volume."""
     while True:
         await asyncio.sleep(_HLS_REAP_EVERY_S)
         try:
@@ -187,8 +187,8 @@ async def _nul_in_text(request: Request, exc: DataError) -> JSONResponse:
     NUL in text, so the driver refuses to even send the query — and a `%00` in a query parameter or
     an escaped NUL in a JSON body (a playlist's `contains=`, a reorder's key list, a collection
     search) was a 500 with a traceback wherever it reached a comparison. Guarding each site did not
-    hold: round 3 guarded the by-rating_key lookup and missed the rest. Any other DataError is left
-    alone — re-raised, it stays the 500 it always was."""
+    hold: an earlier fix guarded the by-rating_key lookup and missed the rest. Any other DataError
+    is left alone — re-raised, it stays the 500 it always was."""
     if "NUL (0x00)" not in str(exc.orig):
         raise exc
     return JSONResponse(