test(plex)+cleanup(quota): S2 code-review fixes (R6 S2)

/code-review high on S2 — 4 findings, 2 fixed, 2 no-change:
- test_plex_sync_txn: a faked PlexClient fails on the second library; asserts the
  first library's row survives (per-section commit). Mutation-checked — moving the
  commit back out of the loop turns it red ([] == ['1']).
- dropped the `[[quota-owns-its-session]]` memory slugs from quota.py docstrings
  (a dangling private-note reference in shipped code; the reasoning is already inline).
No change: record_usage's per-call second connection is a transient spike, fine for
this app's concurrency; the two-site `with SessionLocal()` duplication isn't worth a
helper until a third caller appears.

Gate: ruff clean; DB lane 181 green.

backend/app/quota.py

@@ -126,7 +126,7 @@ def log_action(user_id: int, action: str) -> None:
 
     Writes in its OWN session (like record_usage) — quota accounting is durable independent of the
     caller's transaction and must never commit the caller's session (that would flush a job's
-    partial state mid-run, defeating its own rollback). See [[quota-owns-its-session]]."""
+    partial state mid-run, defeating its own rollback)."""
     with SessionLocal() as qs:
         qs.add(QuotaEvent(user_id=user_id, action=action, units=0))
         qs.commit()
@@ -158,7 +158,7 @@ def record_usage(units: int) -> None:
     back (or is read-only and never commits) must NOT lose the record — under-counting the shared
     daily budget risks overspending the real API quota. Equally, quota must never commit the
     CALLER's session (the old behaviour) — that persisted a job's partial state mid-run behind its
-    own `except: db.rollback()`. See [[quota-owns-its-session]]."""
+    own `except: db.rollback()`."""
     if units <= 0:
         return
     day = pacific_today()