fix(metrics): MetricsFlushScheduler honour ingestion config flush interval

The @Scheduled placeholder read ${ingestion.flush-interval-ms:1000}
(unprefixed) but IngestionConfig binds cameleer.server.ingestion.* —
YAML tuning of the metrics flush interval was silently ignored and the
scheduler fell back to the 1s default in every environment.

Corrected to ${cameleer.server.ingestion.flush-interval-ms:1000}.

(The initial attempt to bind via SpEL #{@ingestionConfig.flushIntervalMs}
failed because beans registered via @EnableConfigurationProperties use a
compound bean name "<prefix>-<FQN>", not the simple camelCase form. The
property-placeholder path is sufficient — IngestionConfig still owns
the Java-side default.)

BackpressureIT: drops the obsolete workaround property
`ingestion.flush-interval-ms=60000`; the single prefixed override now
controls both buffer config and flush cadence.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cameleer-server-app/src/main/java/com/cameleer/server/app/ingestion/MetricsFlushScheduler.java

@@ -30,7 +30,7 @@ public class MetricsFlushScheduler implements SmartLifecycle {
         this.batchSize = config.getBatchSize();
     }
 
-    @Scheduled(fixedDelayString = "${ingestion.flush-interval-ms:1000}")
+    @Scheduled(fixedDelayString = "${cameleer.server.ingestion.flush-interval-ms:1000}")
     public void flush() {
         try {
             List<MetricsSnapshot> batch = metricsBuffer.drain(batchSize);