调整后台文字模型请求发送图片或者视频的方式base64，而不是现在的链接形式，增加后台配置

video-gen-api/app/utils/media.py

@@ -0,0 +1,54 @@
+"""媒体文件 → base64 data URI 转换工具。"""
+import base64
+import mimetypes
+import os
+
+import httpx
+from sqlalchemy import select
+
+from app.config import settings
+from app.models.system_config import SystemConfig
+
+
+async def get_llm_media_as_base64(db=None) -> bool:
+    """读取 SystemConfig 中的 llm_media_as_base64 设置。无 DB 连接时回退到 env。"""
+    if db is not None:
+        try:
+            result = await db.execute(
+                select(SystemConfig).where(SystemConfig.key == "llm_media_as_base64").limit(1)
+            )
+            config = result.scalar_one_or_none()
+            if config:
+                return config.value.lower() in ("true", "1", "yes")
+        except Exception:
+            pass
+    return settings.LLM_MEDIA_AS_BASE64
+
+
+async def media_to_base64(url: str, fallback_mime: str = "image/png", max_mb: int = 20) -> str:
+    """将任意媒体 URL/路径转为 base64 data URI。
+
+    - data: URI → 原样返回
+    - http(s):// → 下载后编码（受 max_mb 限制）
+    - /uploads/xxx 或本地路径 → 读盘编码
+    """
+    if url.startswith("data:"):
+        return url
+
+    if url.startswith(("http://", "https://")):
+        async with httpx.AsyncClient(timeout=60) as client:
+            resp = await client.get(url)
+        size_mb = len(resp.content) / (1024 * 1024)
+        if size_mb > max_mb:
+            raise ValueError(f"媒体文件过大 ({size_mb:.1f}MB > {max_mb}MB)")
+        b64 = base64.b64encode(resp.content).decode()
+        mime = resp.headers.get("content-type") or fallback_mime
+        return f"data:{mime};base64,{b64}"
+
+    # 本地路径
+    relative_path = url.replace("/uploads/", "", 1).lstrip("/")
+    file_path = os.path.join(settings.UPLOAD_LOCAL_PATH, relative_path)
+    mime = mimetypes.guess_type(file_path)[0] or fallback_mime
+    with open(file_path, "rb") as f:
+        b64 = base64.b64encode(f.read()).decode()
+    return f"data:{mime};base64,{b64}"