otel: map journald PRIORITY onto a severity at every journald receiver

Records reached VictoriaLogs carrying the journal's raw PRIORITY and
severity_text "Unspecified" — every line in the store, at every tier, with
no level a query or a dashboard could read. VictoriaLogs has no ingest
parameter naming a level field; it auto-detects one by field name, so the
mapping has to happen in the collector.

A stanza severity_parser on each journald receiver, from one shared file
rather than a copy per tier: the two receivers are unrelated config (a
fixed stanza in the agent container, a parameterised block inside the
swarm-otel container) and a drifted copy fails silently — every line still
arrives, labelled as the wrong thing.

Two details that are easy to get wrong and quiet when wrong. PRIORITY
counts down in urgency where the OTEL severity counts up, so the table is
written as a table. And overwrite_text is required: without it the parser
sets the severity number and leaves the text as the raw digit, so
severity_text arrives as the literal "6" — populated, and not a level
anything renders.

nix/agent-modules/otel.nix

@@ -265,6 +265,14 @@ in
           # and what the harness spawns, so there is no foreign traffic to
           # filter out and an allowlist would only be a list to forget to
           # update.
+
+          # journald's PRIORITY carries the level every line already has;
+          # without this the record reaches VictoriaLogs with
+          # `severity_text: Unspecified` and the store cannot tell an error
+          # from a debug line. The table — and the inverted direction that
+          # makes it worth a file of its own — lives in
+          # ../journald-severity.nix, shared with the swarm tier's receiver.
+          operators = import ../journald-severity.nix;
         };
 
         # Identity the hop above cannot supply. A host-side reader can say