Blink cameras have a new feature called Smart Video Descriptions (SVD) - the camera generates a short AI-written summary of each motion clip, like "A person is walking on the driveway" or "A delivery truck has arrived." These descriptions show up in the official Blink app, but blinkpy currently can't access them because they're only available through the newer v4 media endpoint.

This PR switches the media fetch from the legacy v1 endpoint to the v4 endpoint (POST /api/v4/accounts/{id}/media), which returns richer data including:

• ai_vd — AI video descriptions with both a full and short summary
• cv_detection — Computer vision labels (e.g., ["person"], ["vehicle"])
• smart_video_descriptions — Account-level flag indicating SVD is enabled

The v4 endpoint is what the official Blink app already uses today; the v1 endpoint simply doesn't include these fields.

What changed

• api.py — Added request_videos_v4() function for the v4 media POST endpoint
• sync_module.py — check_new_videos() now calls the v4 endpoint and extracts ai_vd and cv_detection from each media entry. Since v4 doesn't support the ?since= server-side filter that v1 had, client-side time filtering (which already existed via check_new_video_time()) handles this cleanly.
• camera.py — Added ai_description, ai_description_short, and cv_detection attributes. Also fixed a URL-doubling bug: v4 returns absolute clip URLs (unlike v1's relative paths), so the code now checks before prepending base_url.
• tests/test_ai_descriptions.py — 12 new tests covering the v4 API call, AI field extraction (present, absent, empty), camera attribute population, recent clips, and the absolute/relative URL handling.
• tests/test_camera_functions.py — Updated recent_clips expected shape to include the new fields.

Backward compatibility

For accounts without SVD enabled, ai_vd comes back as None and the fields are simply omitted — no errors, no empty strings. The cv_detection and AI description attributes on the camera object default to None until populated, so existing integrations won't break.

Test plan

• pytest — 230 tests pass (218 existing + 12 new)
• ruff check — clean
• black --check — clean
• Tested locally against a live Blink account with SVD enabled