Add OpenSearch search backend with hybrid neural+BM25 support

Extract a SearchBackend trait from the existing RocksDB search code and
add an OpenSearch implementation supporting cross-room search, relevance
ranking, fuzzy matching, English stemming, and optional hybrid
neural+BM25 semantic search using sentence-transformers.

Fix macOS build by gating RLIMIT_NPROC and getrusage to supported
platforms.

src/core/config/check.rs

@@ -116,6 +116,22 @@ pub fn check(config: &Config) -> Result {
 		});
 	}
 
+	if config.search_backend == super::SearchBackendConfig::OpenSearch
+		&& config.search_opensearch_url.is_none()
+	{
+		return Err!(Config(
+			"search_opensearch_url",
+			"OpenSearch URL must be set when search_backend is \"opensearch\""
+		));
+	}
+
+	if config.search_opensearch_hybrid && config.search_opensearch_model_id.is_none() {
+		return Err!(Config(
+			"search_opensearch_model_id",
+			"Model ID must be set when search_opensearch_hybrid is enabled"
+		));
+	}
+
 	// rocksdb does not allow max_log_files to be 0
 	if config.rocksdb_max_log_files == 0 {
 		return Err!(Config(