fix(playlists): a refused push spends none of the push allowance

The cooldown was checked before the list was found pushable and before its sync claim, so Watch later
or a list another sync held used up the push allowance. It now counts only a push that runs. The
docstring that called a YouTube mirror unpushable is corrected: its local edits sync back.

backend/app/routes/playlists.py

@@ -261,8 +261,9 @@ def _check_push_cooldown(user: User) -> None:
 
 
 def _pushable(db: Session, user: User, playlist_id: int) -> "Playlist":
-    """A local playlist the user may push to YouTube (create or update). The built-in
-    Watch later and read-only YouTube mirrors are never pushable."""
+    """A playlist the user may push to YouTube: a local one (created there on its first push) or a
+    mirror of their own YouTube list (its local edits merged back). Only the built-in Watch later is
+    never pushable."""
     pl = _own_playlist(db, user, playlist_id)
     if pl.kind == "watch_later":
         raise _refused(400, "watch_later_not_pushable")
@@ -278,8 +279,8 @@ def push_plan(
     """Dry run: what a two-way 'Sync to YouTube' merge would do (what each side gains/loses) +
     the quota estimate, so the UI can show a confirm. For a linked playlist this reads the live
     YouTube state (cheap, 1 unit/page). Write scope is required — syncing is a YouTube write."""
-    _check_push_cooldown(user)
     pl = _pushable(db, user, playlist_id)
+    _check_push_cooldown(user)  # after the refusal: a list that cannot be pushed spends nothing
     try:
         with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
             plan = plan_merge(db, user, pl)
@@ -299,13 +300,14 @@ def push(
     """Two-way sync a local playlist with YouTube (create it on first push, else 3-way merge so
     neither side loses an item). Refuses if the estimated quota exceeds what's left today, so a
     big sync can't strand half-applied. Write scope required (require_write_scope)."""
-    _check_push_cooldown(user)
     pl = _pushable(db, user, playlist_id)
     # One sync of a list at a time, across tabs, devices and the scheduler (`sync_claim`): the
     # client's own in-flight flag only covers the menus of one page.
     with sync_claim(db, pl.id) as free:
         if not free:
             raise _refused(409, "sync_busy")
+        # Spent only by a push that runs, as Revert's pull: a busy list or Watch later costs nothing.
+        _check_push_cooldown(user)
         try:
             with quota.attribute(user.id, quota.QuotaAction.PLAYLISTS_PUSH):
                 # Read the live playlist ONCE (linked playlists only) and share it with both the