feat: 新增题材页面（列表 + 详情）

- 主页增加「题材热点」入口，跳转题材列表页
- 题材列表页：展示全部题材，支持按涨幅/强度/热度/成交额排序
- 题材详情页：简介、热点事件、相关新闻、板块涨跌统计、全部相关股票（含入选理由默认展开）
- 后端逆向封装东方财富题材接口 getThemeList/getDetail/getStockList，含交易时段感知缓存

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

backend/routes/themes.py

@@ -0,0 +1,53 @@
+"""题材数据路由：题材列表、题材详情、题材相关股票"""
+
+from fastapi import APIRouter, Query, HTTPException
+from fastapi.responses import JSONResponse
+from services import themes
+
+router = APIRouter()
+
+# 上游反代/CDN 可能按 path 缓存，显式禁止缓存
+_NO_CACHE_HEADERS = {"Cache-Control": "no-store, no-cache, must-revalidate, max-age=0"}
+
+
+@router.get("", summary="题材列表")
+async def theme_list(
+    sort_field: int = Query(1, ge=1, le=5, description="排序字段：1=涨幅 3=强度 4=热度排名 5=成交额"),
+    asc: bool = Query(False, description="True=升序, False=降序"),
+):
+    if sort_field not in (1, 3, 4, 5):
+        raise HTTPException(status_code=400, detail="排序字段仅支持 1/3/4/5")
+
+    data = await themes.fetch_theme_list(sort_field, asc)
+    return JSONResponse(
+        {"data": data, "count": len(data), "sort_field": sort_field, "asc": asc},
+        headers=_NO_CACHE_HEADERS,
+    )
+
+
+@router.get("/{theme_code}/detail", summary="题材详情")
+async def theme_detail(theme_code: str):
+    data = await themes.fetch_theme_detail(theme_code)
+    if not data:
+        return JSONResponse(
+            {"data": None, "theme_code": theme_code},
+            headers=_NO_CACHE_HEADERS,
+        )
+    return JSONResponse(
+        {"data": data, "theme_code": theme_code},
+        headers=_NO_CACHE_HEADERS,
+    )
+
+
+@router.get("/{theme_code}/stocks", summary="题材相关股票（全部）")
+async def theme_stocks(theme_code: str):
+    result = await themes.fetch_theme_stocks(theme_code)
+    return JSONResponse(
+        {
+            "data": result.get("stockList", []),
+            "statistic": result.get("statistic", {}),
+            "total": result.get("total", 0),
+            "theme_code": theme_code,
+        },
+        headers=_NO_CACHE_HEADERS,
+    )