feat: 记录并展示每次生成消耗的LLM调用次数（llm_calls）

backend/database.py

@@ -83,6 +83,7 @@ CREATE TABLE IF NOT EXISTS ai_reports (
   model TEXT NOT NULL,
   tokens_used INTEGER,
   generation_count INTEGER NOT NULL DEFAULT 1,
+  llm_calls INTEGER,
   created_at TEXT NOT NULL DEFAULT (datetime('now','localtime')),
   updated_at TEXT,
   UNIQUE(trade_date, report_type)
@@ -121,6 +122,7 @@ def init_db():
     for alter_sql in (
         "ALTER TABLE ai_reports ADD COLUMN generation_count INTEGER NOT NULL DEFAULT 1",
         "ALTER TABLE ai_reports ADD COLUMN updated_at TEXT",
+        "ALTER TABLE ai_reports ADD COLUMN llm_calls INTEGER",
     ):
         try:
             conn.execute(alter_sql)