fix(scheduler,i18n): a failure a job names reads in the reader's language

The audit log showed a failed RSS poll as the server's English sentence
under a Hungarian UI. A job can now raise progress.JobFailed(reason,
message, **params): the scheduler stores the reason and params as the
run's data (dashboard, manual-run notice, audit row) beside the English
line, and the client renders scheduler.failure.<reason>. The RSS outage
names its first channel and the reason code (HTTP 404 vs unreachable).
An upstream's own error text stays as raised (diagnostic).

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

@@ -186,5 +186,14 @@
     "lowquota": "low quota",
     "noreadscope": "no YouTube read access",
     "busy": "another sync is running"
+  },
+  "failure": {
+    "rss_all_failed": "The RSS feed answered for none of the {{total}} channels — the first: {{channel}}, {{cause}}. Read through the API instead: {{read}}"
+  },
+  "rssCause": {
+    "http": "HTTP {{status}}",
+    "transport": "unreachable",
+    "apply": "could not be saved",
+    "error": "unexpected error"
   }
 }

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

@@ -186,5 +186,14 @@
     "lowquota": "kevés a kvóta",
     "noreadscope": "nincs YouTube-olvasási jogosultság",
     "busy": "egy másik szinkron fut"
+  },
+  "failure": {
+    "rss_all_failed": "Az RSS-hírcsatorna egyik csatornánál sem válaszolt ({{total}}) — az első: {{channel}}, {{cause}}. Az API-n át beolvasva helyette: {{read}}"
+  },
+  "rssCause": {
+    "http": "HTTP {{status}}",
+    "transport": "nem érhető el",
+    "apply": "nem sikerült menteni",
+    "error": "váratlan hiba"
   }
 }