refactor(backend): quota.measured helper, consistent admin gating, messageable predicate

- quota.measured() context manager folds the before/after units diff that the
  manual sync routes each re-spelled (also makes quota_remaining_today consistent).
- sync pause/resume now use Depends(admin_user) instead of inline role checks.
- is_messageable_user() unifies the 'real, active, non-suspended human' rule that
  was encoded three ways (WS auth, send recipient, and the SQL _messageable()).

backend/app/quota.py

@@ -66,6 +66,26 @@ def attribute(actor_id: int | None, action: str):
         _action.reset(t2)
 
 
+class Measure:
+    """Result handle from `measured()` — read .used / .remaining after the block exits."""
+
+    used = 0
+    remaining = 0
+
+
+@contextlib.contextmanager
+def measured(db: Session, actor_id: int | None, action: str):
+    """Like `attribute()`, but also reports how much quota the block spent. Yields a `Measure`
+    whose `.used` (units charged inside the block) and `.remaining` (today's remaining after)
+    are populated on exit — so manual sync routes don't each re-spell the before/after diff."""
+    m = Measure()
+    before = units_used_today(db)
+    with attribute(actor_id, action):
+        yield m
+    m.used = units_used_today(db) - before
+    m.remaining = remaining_today(db)
+
+
 def pacific_today():
     return datetime.now(_PACIFIC).date()