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/sync/playlists.py

@@ -168,7 +168,7 @@ def _load_baseline(pl: Playlist) -> tuple[list[str] | None, set[str]]:
 
 def parse_baseline(pl: Playlist) -> list[str] | None:
     """The last-synced ordered id set, or None when no baseline has been recorded yet (a fresh
-    export, or a link created before S7). None makes the first merge union losslessly."""
+    export, or an older link). None makes the first merge union losslessly."""
     return _load_baseline(pl)[0]
 
 
@@ -209,7 +209,7 @@ def _set_baseline_under_lock(
     meanwhile — lock the list and derive `dirty` from what is there, or the edit would read as
     synced and never reach YouTube. The name too: the lock re-reads a rename made meanwhile, and a
     baseline taken over THAT name matched it, so the list read clean with YouTube's title still the
-    old one (E28 review round 4)."""
+    old one."""
     _lock_or_gone(db, pl)
     _set_baseline(pl, ids, unavailable, name=name)
     recompute_dirty(db, pl)
@@ -322,7 +322,7 @@ def user_sync_claim(db: Session, user_id: int) -> AbstractContextManager[bool]:
 def _mirror_listing(yt: YouTubeClient, ytid: str, count: object, pl_id: int) -> list[str] | None:
     """A mirror's video ids from YouTube, or None when there is nothing to believe: the list was
     deleted between the pull's listing and this read (the next pull drops the mirror as vanished —
-    not a reason to fail every other list, E32 review round 2), or YouTube listed no items for a
+    not a reason to fail every other list), or YouTube listed no items for a
     list it counts as non-empty (`blank_listing`)."""
     try:
         ids = list(yt.iter_my_playlist_video_ids(ytid))