feat(01-02): implement ingestion REST controllers with backpressure

- ExecutionController: POST /api/v1/data/executions (single or array)
- DiagramController: POST /api/v1/data/diagrams (single or array)
- MetricsController: POST /api/v1/data/metrics (array)
- All return 202 Accepted or 503 with Retry-After when buffer full
- Fix duplicate IngestionConfig bean (remove @Configuration, use @EnableConfigurationProperties)
- Fix BackpressureIT timing by using batch POST and 60s flush interval
- All 11 integration tests green

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/config/IngestionConfig.java

@@ -1,13 +1,13 @@
 package com.cameleer3.server.app.config;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
-import org.springframework.context.annotation.Configuration;
 
 /**
  * Configuration properties for the ingestion write buffer.
  * Bound from the {@code ingestion.*} namespace in application.yml.
+ * <p>
+ * Registered via {@code @EnableConfigurationProperties} on the application class.
  */
-@Configuration
 @ConfigurationProperties(prefix = "ingestion")
 public class IngestionConfig {