refactor: consolidate oidc_config and admin_thresholds into generic server_config table

Single JSONB key-value table replaces two singleton config tables, making
future config types trivial to add. Also fixes pre-existing IT failures:
Flyway URL not overridden by Testcontainers, threshold test ordering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/test/java/com/cameleer3/server/app/AbstractPostgresIT.java

@@ -42,6 +42,9 @@ public abstract class AbstractPostgresIT {
         registry.add("spring.datasource.password", postgres::getPassword);
         registry.add("spring.datasource.driver-class-name", () -> "org.postgresql.Driver");
         registry.add("spring.flyway.enabled", () -> "true");
+        registry.add("spring.flyway.url", postgres::getJdbcUrl);
+        registry.add("spring.flyway.user", postgres::getUsername);
+        registry.add("spring.flyway.password", postgres::getPassword);
         registry.add("opensearch.url", opensearch::getHttpHostAddress);
     }
 }