fix(playlists): one YouTube sync of a list at a time, across tabs and the scheduler

The client's in-flight flag covers the menus of one page only. A push reads YouTube, then writes
both sides for seconds and commits step by step, so a row lock ends at its first commit: two tabs,
or a push and the scheduled mirror refresh, could interleave into a doubled YouTube item or a
baseline recorded over the other's writes. `sync_claim` takes a session-level advisory lock on a
connection of its own (it survives those commits; a crashed worker's lock dies with its
connection, so no claim column is left stuck). Push and revert answer 409 `sync_busy`; the
scheduled refresh skips a list being synced and picks it up next run.

frontend/src/i18n/locales/en/playlists.json

@@ -86,7 +86,8 @@
     "watch_later_not_pushable": "The Watch later list can't be sent to YouTube",
     "watch_later_undeletable": "The Watch later list can't be deleted",
     "youtube_editing_off": "Enable YouTube editing in Settings first",
-    "quota_exceeded": "Not enough YouTube quota left today for this sync"
+    "quota_exceeded": "Not enough YouTube quota left today for this sync",
+    "sync_busy": "This playlist is already syncing with YouTube — try again when it finishes"
   },
   "moveErrors": {
     "not_in_playlist": "That video isn't in this playlist any more — it changed somewhere else",

frontend/src/i18n/locales/hu/playlists.json

@@ -86,7 +86,8 @@
     "watch_later_not_pushable": "A „Megnézem később” listát nem lehet a YouTube-ra küldeni",
     "watch_later_undeletable": "A „Megnézem később” lista nem törölhető",
     "youtube_editing_off": "Előbb engedélyezd a YouTube-szerkesztést a Beállításokban",
-    "quota_exceeded": "Mára nem maradt elég YouTube-keret ehhez a szinkronhoz"
+    "quota_exceeded": "Mára nem maradt elég YouTube-keret ehhez a szinkronhoz",
+    "sync_busy": "Ez a lista épp szinkronizál a YouTube-bal — próbáld újra, ha végzett"
   },
   "moveErrors": {
     "not_in_playlist": "Ez a videó már nincs ebben a listában — időközben máshol megváltozott",