When add_from_reader() returns after prebuffering, the file may not
exist on disk yet due to tokio::spawn() scheduling. This caused
"Cache entry not found" errors when playlist tried to validate the pk.
Solution:
- If DB entry exists but file doesn't, wait up to 1 second for file creation
- This handles the race condition between prebuffer completion and
File::create() in the background task
- Deterministic and robust: either file exists or we timeout with error
The fix preserves the progressive caching design while ensuring
validation is deterministic.
Test: Verified no "Cache entry not found" errors with clean cache.
cdf24b0143