feat(US3+4): video frame extraction + video-to-text — POST /api/v1/video/*

- app/models/video_models.py: ExtractFramesRequest, VideoToTextRequest,
  FrameInfo, VideoJobCallback, VideoAcceptedResponse
- app/services/video_service.py: interval+keyframe frame extraction,
  uniform-sample video-to-text, HTTP callback, temp file cleanup
- app/routers/video.py: size check helper (_check_video_size via head_object),
  BackgroundTasks enqueue for both endpoints
- tests: 6 service + 4 router tests, 10/10 passing

app/routers/video.py

@@ -1,3 +1,69 @@
-from fastapi import APIRouter
+from fastapi import APIRouter, BackgroundTasks, Depends
+
+from app.clients.llm.base import LLMClient
+from app.clients.storage.base import StorageClient
+from app.core.config import get_config
+from app.core.dependencies import get_llm_client, get_storage_client
+from app.core.exceptions import VideoTooLargeError
+from app.models.video_models import (
+    ExtractFramesRequest,
+    VideoAcceptedResponse,
+    VideoToTextRequest,
+)
+from app.services import video_service
 
 router = APIRouter(tags=["Video"])
+
+
+async def _check_video_size(storage: StorageClient, bucket: str, file_path: str, max_mb: int) -> None:
+    size_bytes = await storage.get_object_size(bucket, file_path)
+    if size_bytes > max_mb * 1024 * 1024:
+        raise VideoTooLargeError(
+            f"视频文件大小超出限制（最大 {max_mb}MB，当前 {size_bytes // 1024 // 1024}MB）"
+        )
+
+
+@router.post("/video/extract-frames", response_model=VideoAcceptedResponse, status_code=202)
+async def extract_frames(
+    req: ExtractFramesRequest,
+    background_tasks: BackgroundTasks,
+    storage: StorageClient = Depends(get_storage_client),
+) -> VideoAcceptedResponse:
+    cfg = get_config()
+    bucket = cfg["storage"]["buckets"]["source_data"]
+    max_mb = cfg["video"]["max_file_size_mb"]
+    callback_url = cfg.get("backend", {}).get("callback_url", "")
+
+    await _check_video_size(storage, bucket, req.file_path, max_mb)
+
+    background_tasks.add_task(
+        video_service.extract_frames_task,
+        req,
+        storage,
+        callback_url,
+    )
+    return VideoAcceptedResponse(message="任务已接受，后台处理中", job_id=req.job_id)
+
+
+@router.post("/video/to-text", response_model=VideoAcceptedResponse, status_code=202)
+async def video_to_text(
+    req: VideoToTextRequest,
+    background_tasks: BackgroundTasks,
+    storage: StorageClient = Depends(get_storage_client),
+    llm: LLMClient = Depends(get_llm_client),
+) -> VideoAcceptedResponse:
+    cfg = get_config()
+    bucket = cfg["storage"]["buckets"]["source_data"]
+    max_mb = cfg["video"]["max_file_size_mb"]
+    callback_url = cfg.get("backend", {}).get("callback_url", "")
+
+    await _check_video_size(storage, bucket, req.file_path, max_mb)
+
+    background_tasks.add_task(
+        video_service.video_to_text_task,
+        req,
+        llm,
+        storage,
+        callback_url,
+    )
+    return VideoAcceptedResponse(message="任务已接受，后台处理中", job_id=req.job_id)