feature: 新增小说封面图片生成功能

backend/app/services/cover_providers/gemini_cover_provider.py

@@ -0,0 +1,78 @@
+"""Gemini 封面图片 Provider"""
+from __future__ import annotations
+
+import base64
+from typing import Any
+
+import httpx
+
+from app.logger import get_logger
+from app.services.cover_providers.base_cover_provider import BaseCoverProvider, CoverGenerationResult
+
+logger = get_logger(__name__)
+
+
+class GeminiCoverProvider(BaseCoverProvider):
+    """基于 Gemini API 的封面生成实现"""
+
+    def __init__(self, api_key: str, base_url: str):
+        self.api_key = api_key
+        self.base_url = (base_url or "https://generativelanguage.googleapis.com/v1beta").rstrip("/")
+
+    async def generate_cover(
+        self,
+        *,
+        prompt: str,
+        model: str,
+        width: int,
+        height: int,
+    ) -> CoverGenerationResult:
+        url = f"{self.base_url}/models/{model}:generateContent?key={self.api_key}"
+        payload: dict[str, Any] = {
+            "contents": [{
+                "role": "user",
+                "parts": [{
+                    "text": (
+                        f"{prompt}\n\n"
+                        f"Generate a final cover image at {width}x{height} pixels. "
+                        "Return one final cover image."
+                    )
+                }]
+            }],
+            "generationConfig": {
+                "temperature": 0.4,
+            },
+        }
+
+        async with httpx.AsyncClient(timeout=120.0) as client:
+            response = await client.post(url, json=payload)
+            response.raise_for_status()
+            data = response.json()
+
+        candidates = data.get("candidates") or []
+        if not candidates:
+            raise ValueError("Gemini 未返回候选结果")
+
+        parts = candidates[0].get("content", {}).get("parts", [])
+        for part in parts:
+            inline_data = part.get("inlineData")
+            if not inline_data:
+                continue
+
+            mime_type = inline_data.get("mimeType", "image/png")
+            image_data = inline_data.get("data")
+            if not image_data:
+                continue
+
+            file_extension = "png" if "png" in mime_type else "jpg"
+            return {
+                "content": base64.b64decode(image_data),
+                "mime_type": mime_type,
+                "file_extension": file_extension,
+                "revised_prompt": None,
+                "provider": "gemini",
+                "model": model,
+            }
+
+        logger.error("Gemini 返回内容中未找到 inlineData 图像数据")
+        raise ValueError("Gemini 未返回图片数据")