feat: add POST /api/v1/probe endpoint

- Add ProbeRequest/ProbeResponse structures
- Support relative and absolute paths
- Cache probe.json for repeated requests
- Return video metadata (uuid, duration, width, height, fps)
- Include cached flag to indicate cache hit
- Export FormatInfo and StreamInfo from probe module
- Update API_ENDPOINTS.md documentation

src/core/probe/mod.rs

@@ -1,3 +1,3 @@
-pub mod probe;
+pub mod ffprobe;
 
-pub use probe::{probe_video, ProbeResult};
+pub use ffprobe::{probe_video, FormatInfo, ProbeResult, StreamInfo};