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/subscriptions.py

@@ -123,7 +123,7 @@ def should_prune(fetched_count: int, existing_count: int) -> bool:
 def vanished[K, V](existing: dict[K, V], listed: Collection[K]) -> list[V]:
     """The local mirrors whose key the remote listing no longer has — none at all when the listing
     is an empty answer that contradicts them (`should_prune`). The YouTube playlist mirrors use it:
-    without it, one blip dropped every mirror (E32 S2)."""
+    without it, one blip dropped every mirror."""
     if not should_prune(len(listed), len(existing)):
         return []
     return [row for key, row in existing.items() if key not in listed]
@@ -132,7 +132,7 @@ def vanished[K, V](existing: dict[K, V], listed: Collection[K]) -> list[V]:
 def blank_listing(listed: Collection, count: object) -> bool:
     """The same rule one level down: no members from a container whose own listing counts some (a
     playlist's `itemCount`) is a blip, not a list emptied. Replacing or merging on it deleted every
-    mirrored item and reset the merge baseline (E32 review)."""
+    mirrored item and reset the merge baseline."""
     return not listed and isinstance(count, int) and count > 0