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).

backend/app/progress.py

@@ -24,6 +24,19 @@ def report(current: int, total: int | None = None, phase: str | None = None) ->
         sink(current, total, phase)
 
 
+class JobFailed(RuntimeError):
+    """A job's own failure, said so the reader's language can name it (E31): the client reads
+    `scheduler.failure.<reason>` and interpolates `params` (keep them to values the reader may see —
+    numbers, ids, short codes). The message is the English line the log and `last_error` keep. An
+    upstream's own error (a timeout, a refused connection) is not one of these: it stays as raised,
+    being diagnostic."""
+
+    def __init__(self, reason: str, message: str, **params: object):
+        super().__init__(message)
+        self.reason = reason
+        self.params = params
+
+
 class bind:
     """Context manager that binds a progress sink for the duration of a job run."""