add explain module

ios/OralTrainer/Services/SentenceBoundaryAPI.swift

@@ -37,3 +37,46 @@ enum SentenceBoundaryAPI {
         return result.value
     }
 }
+
+struct SentenceExplanationResponse: Decodable {
+    let explanation: String
+
+    private enum CodingKeys: String, CodingKey {
+        case explanation
+    }
+}
+
+extension SentenceBoundaryAPI {
+    static func explain(
+        _ sentence: SentenceBoundary,
+        courseTitle: String
+    ) async throws -> String {
+        var body: [String: Any] = [
+            "sentence_text": sentence.text ?? "",
+            "course_title": courseTitle,
+            "sentence_index": sentence.index,
+        ]
+        if let language = sentence.language {
+            body["language"] = language
+        }
+        var headers: [String: String] = ["Content-Type": "application/json"]
+        if !AppConfig.authToken.isEmpty {
+            headers["Authorization"] = "Bearer \(AppConfig.authToken)"
+        }
+        let result = try await HTTPClient.execute(
+            baseURL: AppConfig.serverBaseURL,
+            path: "api/v1/sentences/explain",
+            method: "POST",
+            timeout: AppConfig.readTimeout,
+            headers: headers,
+            body: try JSONSerialization.data(withJSONObject: body)
+        )
+        guard (200...299).contains(result.statusCode) else {
+            let detail = String(data: result.body, encoding: .utf8) ?? ""
+            throw HTTPClientError.badStatus(result.statusCode, detail)
+        }
+        return try JSONDecoder()
+            .decode(SentenceExplanationResponse.self, from: result.body)
+            .explanation
+    }
+}