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

tests/test_video_router.py

@@ -0,0 +1,71 @@
+import pytest
+from unittest.mock import AsyncMock, patch
+
+from app.core.exceptions import VideoTooLargeError
+
+
+def test_extract_frames_returns_202(client, mock_storage):
+    mock_storage.get_object_size = AsyncMock(return_value=10 * 1024 * 1024)  # 10 MB
+
+    with patch("app.routers.video.BackgroundTasks.add_task"):
+        resp = client.post(
+            "/api/v1/video/extract-frames",
+            json={
+                "file_path": "video/test.mp4",
+                "source_id": 10,
+                "job_id": 42,
+            },
+        )
+    assert resp.status_code == 202
+    data = resp.json()
+    assert data["job_id"] == 42
+
+
+def test_extract_frames_video_too_large_returns_400(client, mock_storage):
+    mock_storage.get_object_size = AsyncMock(return_value=300 * 1024 * 1024)  # 300 MB > 200 MB
+
+    resp = client.post(
+        "/api/v1/video/extract-frames",
+        json={
+            "file_path": "video/big.mp4",
+            "source_id": 10,
+            "job_id": 99,
+        },
+    )
+    assert resp.status_code == 400
+    assert resp.json()["code"] == "VIDEO_TOO_LARGE"
+
+
+def test_video_to_text_returns_202(client, mock_storage):
+    mock_storage.get_object_size = AsyncMock(return_value=10 * 1024 * 1024)
+
+    with patch("app.routers.video.BackgroundTasks.add_task"):
+        resp = client.post(
+            "/api/v1/video/to-text",
+            json={
+                "file_path": "video/test.mp4",
+                "source_id": 10,
+                "job_id": 43,
+                "start_sec": 0,
+                "end_sec": 60,
+            },
+        )
+    assert resp.status_code == 202
+    assert resp.json()["job_id"] == 43
+
+
+def test_video_to_text_too_large_returns_400(client, mock_storage):
+    mock_storage.get_object_size = AsyncMock(return_value=300 * 1024 * 1024)
+
+    resp = client.post(
+        "/api/v1/video/to-text",
+        json={
+            "file_path": "video/big.mp4",
+            "source_id": 10,
+            "job_id": 99,
+            "start_sec": 0,
+            "end_sec": 60,
+        },
+    )
+    assert resp.status_code == 400
+    assert resp.json()["code"] == "VIDEO_TOO_LARGE"