refactor(plex): address /code-review on the R12 S1 split

Cleanup only, no behaviour change (ruff clean + pytest 193):
- imports hoisted above `router =` and grouped/sorted (isort) across the split
  files; ruff-format pass restores the 2-blank-line PEP8 spacing the generator
  collapsed to 1.
- the shared on-disk cache root moves to a neutral app/plex/cache.py, so
  subtitles.py no longer imports it from imgcache.py (drops the cross-import).
- the former plex.py module docstring's access-model note (read endpoints open
  to any authed user; admin endpoints test/sync) is restored in the package
  __init__.py.

backend/app/plex/rollup.py

@@ -2,6 +2,7 @@
 
 Drives the show/season grid badges and the Resume / Play-from-start / mark-all buttons.
 """
+
 from sqlalchemy import and_, case, func, or_
 from sqlalchemy.orm import Session, aliased
 
@@ -23,7 +24,11 @@ def rollup(cards: list[dict]) -> dict:
     the episode count. Drives the show/season Resume + Play + mark-all buttons."""
     total = len(cards)
     watched = sum(1 for c in cards if c.get("status") == "watched")
-    inprog = [c for c in cards if (c.get("position_seconds") or 0) > 0 and c.get("status") != "watched"]
+    inprog = [
+        c
+        for c in cards
+        if (c.get("position_seconds") or 0) > 0 and c.get("status") != "watched"
+    ]
     resume = None
     if inprog:
         resume = inprog[-1]  # continue the latest-started episode
@@ -49,7 +54,10 @@ def agg_subquery(db: Session, user_id: int):
             func.count(
                 case(
                     (
-                        and_(ps.position_seconds > 0, or_(ps.status.is_(None), ps.status != "watched")),
+                        and_(
+                            ps.position_seconds > 0,
+                            or_(ps.status.is_(None), ps.status != "watched"),
+                        ),
                         1,
                     )
                 )