Enforce deterministic gateway task policies

Make the server authoritative for thinking, model, search, tools, retry, and output budgets while preserving legacy client behavior.

src/main/kotlin/com/osglab/account/config/AppConfig.kt

@@ -104,6 +104,10 @@ data class AppConfig(
                     "app.storeKit.products is required when StoreKit is enabled"
                 }
             }
+            val deepSeekModel = config.valueOrDefault(
+                "app.providers.deepseek.model",
+                "deepseek-v4-flash",
+            )
             val providers = ProvidersConfig(
                 volcengine = VolcengineConfig(
                     endpoint = config.valueOrDefault(
@@ -124,7 +128,9 @@ data class AppConfig(
                         "https://api.deepseek.com/v1",
                     ),
                     apiKey = config.optionalValue("app.providers.deepseek.apiKey"),
-                    model = config.valueOrDefault("app.providers.deepseek.model", "deepseek-v4-flash"),
+                    model = deepSeekModel,
+                    reasoningModel = config.optionalValue("app.providers.deepseek.reasoningModel")
+                        ?: deepSeekModel,
                 ),
             )
             val integrity = IntegrityConfig(
@@ -411,6 +417,7 @@ data class DeepSeekConfig(
     val endpoint: String,
     val apiKey: String?,
     val model: String,
+    val reasoningModel: String = model,
 ) {
     val credentialsAvailable: Boolean
         get() = !apiKey.isNullOrBlank()