Fix watch folder startup race

[brianmeyer]

Summary

• capture the initial watch snapshot synchronously in start_watch before the scanner thread begins
• have the scanner loop reuse that baseline instead of rebuilding it later
• replace fixed sleeps in the watch-folder media tests with bounded polling so CI checks behavior instead of scheduler timing

Why

Files created immediately after start_watch could be absorbed into the scanner's initial baseline instead of emitting a created event. That meant document/image ingest could be skipped even though the later delete was observed.

Testing

• pytest -q tests/test_watch_folder.py -q
• pytest -q tests/test_search_batch.py tests/test_storage.py -q

[brianmeyer]

Superseded by #21, which carries the watch-folder startup race fix and passed CI before merge.