Implement Phase 1 task 1.10 (Prometheus metrics + /healthz + /readyz)

Replaces the placeholder Metrics shim with a prom-client implementation
in src/observability/metrics.ts: all 10 Phase 1 metrics from the wiki
spec, plus nodejs_* defaults. Exposes /metrics, /healthz, /readyz over
node:http on METRICS_PORT (9090); /readyz returns 503 when Redis status
is not 'ready' or the TCP listener isn't bound.

The Metrics interface in src/core/types.ts is unchanged — adapter call
sites continue to use the same inc/observe shape. Only main.ts sees the
extended type that adds serializeMetrics().

Side effects:
- Dockerfile re-enables HEALTHCHECK pointing at /readyz, and EXPOSE 9090.
- frame-ingested log downgraded back to debug now that
  teltonika_records_published_total is scrapeable.
- 19 new unit tests covering exposition format, all metric types, and
  every HTTP endpoint path. Total now 98 passing.

Note: deploy/compose.yaml still does not expose 9090 — separate decision
about how Prometheus reaches the service (host port vs. internal scraper
on the same Docker network).

src/adapters/teltonika/index.ts

@@ -217,9 +217,9 @@ export function createTeltonikaAdapter(options: TeltonikaAdapterOptions): Adapte
           result: 'ok',
         });
 
-        // Pilot-stage visibility into per-frame ingest. Downgrade to debug
-        // once task 1.10 wires up prom-client and frames_total is scrapeable.
-        sessionLogger.info(
+        // teltonika_frames_total{result="ok"} and teltonika_records_published_total
+        // now carry this signal in Prometheus; keep the log at debug to avoid noise.
+        sessionLogger.debug(
           { codec: codecLabel, records: result.recordCount },
           'frame ingested',
         );