Sweep() used to run synchronously at startup without any time bound.
On a large uploads directory this can block the server for 30s or more,
keeping /healthz returning 503 while the reaper works. Add a context
timeout so startup continues if the sweep cannot finish within budget.
The timeout is set to 5 seconds: long enough to clear typical leftover
state, short enough that a stuck sweep will not meaningfully delay
health checks or startup readiness.
Change Store.Sweep to accept a context.Context and check ctx.Err() at
the top of each iteration. If the context is cancelled or times out,
Sweep returns the files deleted so far and ctx.Err().
Add TestStore_Sweep_HonorsContextCancel to ensure a cancelled context
causes Sweep to exit early instead of running to completion.
Deliberately unchanged: there is still no periodic reaper. The server
only sweeps once at startup; this change just bounds that single sweep.
Co-Authored-By: Claude <noreply@anthropic.com>
