feat(channel-monitor): 添加 API 模式迁移

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/migrations/138_channel_monitor_openai_api_mode.sql

@@ -0,0 +1,40 @@
+-- Migration: 137_channel_monitor_openai_api_mode
+-- 为渠道监控和请求模板增加 OpenAI 协议模式：
+--   chat_completions -> /v1/chat/completions + messages
+--   responses        -> /v1/responses + instructions/input
+-- 历史数据默认保持 chat_completions，避免改变现有监控行为。
+
+ALTER TABLE channel_monitors
+    ADD COLUMN IF NOT EXISTS api_mode VARCHAR(32) NOT NULL DEFAULT 'chat_completions';
+
+ALTER TABLE channel_monitor_request_templates
+    ADD COLUMN IF NOT EXISTS api_mode VARCHAR(32) NOT NULL DEFAULT 'chat_completions';
+
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM information_schema.table_constraints
+        WHERE constraint_name = 'channel_monitors_api_mode_check'
+          AND table_name = 'channel_monitors'
+    ) THEN
+        ALTER TABLE channel_monitors
+            ADD CONSTRAINT channel_monitors_api_mode_check
+            CHECK (api_mode IN ('chat_completions', 'responses'));
+    END IF;
+
+    IF NOT EXISTS (
+        SELECT 1 FROM information_schema.table_constraints
+        WHERE constraint_name = 'channel_monitor_request_templates_api_mode_check'
+          AND table_name = 'channel_monitor_request_templates'
+    ) THEN
+        ALTER TABLE channel_monitor_request_templates
+            ADD CONSTRAINT channel_monitor_request_templates_api_mode_check
+            CHECK (api_mode IN ('chat_completions', 'responses'));
+    END IF;
+END $$;
+
+CREATE INDEX IF NOT EXISTS idx_channel_monitors_provider_api_mode
+    ON channel_monitors (provider, api_mode);
+
+CREATE INDEX IF NOT EXISTS idx_channel_monitor_templates_provider_api_mode
+    ON channel_monitor_request_templates (provider, api_mode);