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/plex/sync.py

@@ -12,7 +12,7 @@ on a NON-empty listing — a section that returns zero items is left untouched,
 glitch that yields an empty page can't wipe a library (the metadata is cheap to re-sync; an accidental
 mass-delete is the only real risk). Library-level reconcile (dropping a whole removed section, the
 dead `enabled` column) is a separate follow-up. Cast + intro/credit markers are fetched lazily on the
-item-detail/player path (P2), not here.
+item-detail/player path, not here.
 """
 
 from datetime import datetime, timezone
@@ -150,7 +150,7 @@ _GUID_PREFERENCE = ("tmdb", "imdb", "tvdb")
 
 
 def _canonical_guid(meta: dict) -> str | None:
-    """The canonical external id used to pair an EN title with its HU-overlay twin (E5 S5). Reads the
+    """The canonical external id used to pair an EN title with its HU-overlay twin. Reads the
     `Guid` array (present only with includeGuids=1 in the listing), preferring tmdb > imdb > tvdb,
     normalised to '<provider>:<id>' (e.g. 'tmdb:12345'). None when no known provider id is present."""
     found: dict[str, str] = {}
@@ -419,7 +419,7 @@ def _paginate(plex: PlexClient, key: str, item_type: int, progress: dict | None
         # A truthy `total` we reached = a trustworthy complete listing. `total` unknown/zero-with-rows
         # (a Plex that doesn't report totalSize) is treated as NOT complete, so prune stays conservative.
         # `total` itself is kept for a caller to whom a server that SAID 0 is news it can trust (the
-        # watch reconcile); an empty mirror or overlay must not read it so (E32 S2).
+        # watch reconcile); an empty mirror or overlay must not read it so.
         progress["complete"] = bool(total) and start >= total
         progress["total"] = total
 
@@ -447,7 +447,7 @@ def _apply_facet_fields(row, meta: dict) -> None:
     # full cast (up to 24, grown by the rich backfill/info-page enrich), silently breaking the
     # person-page cast filter for names past the top ~20. The watermark gate below (`people_enriched_at
     # is None`) normally protects this, but it is NOT enough on its own: a migration that resets the
-    # watermark re-opens the regression (0065 nulled it, which is exactly how the S5b fix got undone in
+    # watermark re-opens the regression (0065 nulled it, which is exactly how a fix got undone in
     # prod). So guard STRUCTURALLY too, by LENGTH against the listing cap: a stored cast LONGER than the
     # cap can only be an enriched fuller list, so keep it (robust to any watermark reset). A stored cast
     # at or below the cap is itself listing-sourced, so take the fresh listing — which tracks Plex cast
@@ -694,7 +694,7 @@ def resync_collection(db: Session, plex: PlexClient, lib: PlexLibrary, rk: str)
 def _believed_children(plex: PlexClient, rk: str, meta: dict) -> list[dict] | None:
     """A collection's members, or None when the answer is not believed: no members from a collection
     whose own listing counts some (`childCount`) is a glitch, not a collection emptied — the same
-    empty-answer rule as the collection listing's (E32 review). Rebuilding on it showed the
+    empty-answer rule as the collection listing's. Rebuilding on it showed the
     collection empty until the next sync."""
     children = plex.collection_children(rk)
     count = str(meta.get("childCount") or "0")
@@ -745,7 +745,7 @@ def _sync_collections(db: Session, plex: PlexClient, lib: PlexLibrary, stats: di
     if not cols and existing:
         # The catalog's own rule (see the module doc): an empty listing while we hold rows is a Plex
         # glitch, not a library emptied of collections. Reconciling on it dropped every collection
-        # (its user-set `editable` flag with it) and every member's keys (E32 S2).
+        # (its user-set `editable` flag with it) and every member's keys.
         log.warning(
             "Plex listed no collections for a library that has some; keeping them this run",
             extra=logs.event("plex.sync.collections_empty", library=lib.plex_key),