fix(inference): isolate video and camera processing sessions #17

Background

Image requests were made request-local in PR #13, but inference/main.py still uses instance-wide self.data, self.paths, and self.recording for video/camera streaming. Overlapping sessions may overwrite downloads, output AVI/MP4 files, detection metadata, progress messages, or stop each other's capture.

Proposed acceptance criteria

• Define explicit per-video/per-camera session identifiers and independent temporary directories (or reject overlaps safely until sessions are supported).
• Keep progress events scoped to the correct client/session rather than broadcast session-specific results.
• Ensure stopCamera stops only the intended recording; clean up capture/writer/files on normal stop and exceptions.
• Avoid shell=True for FFmpeg invocation; handle failed transcoding and missing outputs explicitly.
• Add deterministic concurrency/cancellation/error-path tests using fake frames and model objects.
• Exercise a representative real video and local webcam workflow end-to-end; document host limitations.

Current status: Image concurrency unit tests pass; video/camera concurrency remains unverified. Do not claim multi-user streaming support until this issue is resolved.