[AZ-178] Implement streaming video detection endpoint

- Added `/detect/video` endpoint for true streaming video detection, allowing inference to start as upload bytes arrive.
- Introduced `run_detect_video_stream` method in the inference module to handle video processing from a file-like object.
- Updated media hashing to include a new function for computing hashes directly from files with minimal I/O.
- Enhanced documentation to reflect changes in video processing and API behavior.

Made-with: Cursor

_docs/03_implementation/implementation_report_video_prewrite.md

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+# Implementation Report — Video Pre-Write Removal
+
+**Date**: 2026-04-01
+**Tasks**: 1
+**Batches**: 1
+**Total Complexity**: 2 points
+
+## Summary
+
+Removed redundant synchronous video pre-writes in the `/detect` endpoint. Videos are now written to disk exactly once by `inference.pyx`'s background writer thread, concurrently with frame detection.
+
+## Tasks Completed
+
+| Task | Name | Complexity | Status |
+|------|------|-----------|--------|
+| AZ-177 | remove_redundant_video_prewrite | 2 | Done |
+
+## Changes
+
+| File | Change |
+|------|--------|
+| src/main.py | Auth'd path: wrap file write with `if kind == "image"` so videos skip pre-write |
+| src/main.py | Non-auth'd path: remove `open().write()` after `mkstemp` — inference handles the write |
+
+## Test Results
+
+18/18 tests passed. 4 new tests added:
+- `test_auth_video_storage_path_opened_wb_once`
+- `test_non_auth_temp_video_opened_wb_once_and_removed`
+- `test_auth_image_still_writes_once_before_detect`
+- `test_video_writer_runs_in_separate_thread_from_executor`