Fix slow retry replaying full fast-retry cycle (~34s lag)

[stef-k]

Summary

• Slow retry previously reset to _fetchWithRetry(attempt=0), replaying a full 5-attempt fast-retry cycle on each poll — causing ~34s lag between visible attempts and 6 per-IP budget hits per cycle
• Now uses _slowRetryOnce: single fetch per slow poll (~15s interval with jitter), reschedule on 503, 1 budget hit per poll
• Permanent errors (404, 500) still fail immediately in slow phase
• Network errors reschedule another slow poll (transient)

Closes #206

Test plan

• dotnet build passes (0 errors)
• dotnet test passes (1407 tests)
• Cold-cache zoom 18→10: gray tiles should fill in at ~15s intervals (not ~34s)
• Panning/zooming away cancels slow retry timers