Fix tile cache stale revalidation

[stef-k]

Summary

• Adds stale-while-revalidate tile cache behavior so stale cached tiles can be served immediately while refresh work runs in the background.
• Aligns Trip Editor tile retry/concurrency slot accounting so retries release and reacquire capacity consistently with the stale-refresh path.

Closes #316.

Validation

• npm run build passed, existing Vite chunk-size warning.
• focused tile tests passed, 65/65.
• dotnet test passed, 1557/1557.
• LOC check passed with warnings only, no hard failures.

Manual validation gap

• No browser/manual repeated-map-visit check was run.
• Frontend slot-accounting path was validated by inspection and build, not direct client test, because no suitable frontend unit harness exists.