Add slow retry phase for tiles exhausting fast retries on 503

[stef-k]

Summary

• After 5 fast retries with exponential backoff exhaust on 503, tiles now enter indefinite 30-second polling instead of going permanently gray
• Each slow-phase trigger resets the attempt counter, giving the tile a fresh fast-retry cycle
• Jitter (±25%) on slow retries prevents synchronized bursts
• Network errors also get slow retry (transient); 404 and other HTTP errors remain permanent failures
• Ensures all tiles eventually load once the per-IP sliding-window budget decays (~60s)

Closes #206

Test plan

• dotnet build passes (0 errors)
• dotnet test passes (1407 tests)
• Cold-cache test at zoom 17-18: all tiles should eventually load (some may take 30-60s for slow retry to kick in)
• Panning/zooming away cancels slow retry timers (no leaked timers)
• 404 tiles still show as permanent gray (no slow retry)