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/youtube/client.py

@@ -135,7 +135,7 @@ def refusal_reason(exc: YouTubeError, *, playlist: bool = False) -> YouTubeRefus
     `playlist`: the action was on ONE playlist the user picked, so a missing playlist is that list,
     gone (deleted on YouTube) — neither a retry nor a reconnect helps. Anywhere else the playlist
     is one the user never named (a channel's uploads, one list of a bulk pull), and "this playlist
-    no longer exists" would describe nothing on their screen (E32 review round 2)."""
+    no longer exists" would describe nothing on their screen."""
     if is_quota_refusal(exc):
         return "quota"
     if playlist and is_playlist_gone(exc):

backend/app/youtube/search_scrape.py

@@ -73,7 +73,7 @@ def _get_config(client: httpx.Client) -> tuple[str, str]:
     The fallback pair is never cached: it used to be, so one failed fetch (or a consent page with no
     keys on it) at the wrong moment pinned every later search to a hard-coded key and version for
     the life of the process — and when those went stale, search returned nothing, with no error
-    anywhere (E32 S2). Now the next search simply tries the live pair again."""
+    anywhere. Now the next search simply tries the live pair again."""
     global _config
     if _config is not None:
         return _config