Fix blocklist import: detect UnknownJailException and abort early

_is_not_found_error in jail_service did not match the concatenated form
'unknownjailexception' that fail2ban produces when it serialises
UnknownJailException, so JailOperationError was raised instead of
JailNotFoundError and every ban attempt in the import loop failed
individually, skipping all 27 840 IPs before returning an error.

Two changes:
- Add 'unknownjail' to the phrase list in _is_not_found_error so that
  UnknownJailException is correctly mapped to JailNotFoundError.
- In blocklist_service.import_source, catch JailNotFoundError explicitly
  and break out of the loop immediately with a warning log instead of
  retrying on every IP.

backend/app/services/blocklist_service.py

@@ -304,6 +304,16 @@ async def import_source(
         try:
             await jail_service.ban_ip(socket_path, BLOCKLIST_JAIL, stripped)
             imported += 1
+        except jail_service.JailNotFoundError as exc:
+            # The target jail does not exist in fail2ban — there is no point
+            # continuing because every subsequent ban would also fail.
+            ban_error = str(exc)
+            log.warning(
+                "blocklist_jail_not_found",
+                jail=BLOCKLIST_JAIL,
+                error=str(exc),
+            )
+            break
         except Exception as exc:
             skipped += 1
             if ban_error is None: