feat: 代理支持、外部API增强、调度器修复、每日文章看板

- 添加 HTTP 代理支持（国内直连、外网走代理）
- 外部 API 新增全文搜索、源健康度/错误筛选、未读筛选
- 修复 APScheduler 线程静默崩溃（_safe_fetch 异常保护）
- 健康检查暴露调度器状态
- Dashboard 新增每日文章数柱状图（按 published_at）
- 文章列表 API 补上 content 字段，日期筛选修复时间范围
- 修复外部 API 双重 external 前缀
- User-Agent 改为 Chrome 标识缓解 403
- 添加完整 API 接口文档

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/routers/dashboard.py

@@ -1,6 +1,7 @@
 """仪表盘统计 API"""
 from fastapi import APIRouter, Depends
 from sqlalchemy.orm import Session
+from sqlalchemy import func, text
 from database import get_db
 from health_checker import get_overall_stats, get_feed_health
 
@@ -31,6 +32,25 @@ def dashboard_health(
     return {"total": total, "items": items}
 
 
+@router.get("/articles-daily")
+def articles_daily(days: int = 30, db: Session = Depends(get_db)):
+    """按发布日期统计文章数量"""
+    from models import Article
+    sql = text("""
+        SELECT DATE(published_at) as date, COUNT(*) as count
+        FROM articles
+        WHERE published_at IS NOT NULL
+          AND published_at >= DATE('now', '-' || :days || ' days')
+        GROUP BY DATE(published_at)
+        ORDER BY date DESC
+    """)
+    rows = db.execute(sql, {"days": days}).fetchall()
+    return {
+        "days": days,
+        "data": [{"date": str(r[0]), "count": r[1]} for r in rows],
+    }
+
+
 @router.get("/recent-activity")
 def recent_activity(limit: int = 20, db: Session = Depends(get_db)):
     """最近的抓取活动"""