Fix tile cache purge timeout on large caches

[stef-k]

Summary

Closes #207

• Converts synchronous DeleteLruCache and DeleteAllMapTileCache admin actions to background Task.Run operations, returning HTTP 202 Accepted immediately
• Adds atomic _purgeInProgress concurrency guard (Interlocked.CompareExchange) — second purge request returns 409 Conflict; tile-provider-change auto-purge skips gracefully
• Broadcasts real-time progress via SSE (started/progress/completed/failed events) on admin-tile-cache-purge channel
• Reduces _cacheLock file-delete chunk size from 100 to 10 with Task.Yield() between chunks to minimize CacheTileAsync writer starvation during large purges
• Admin UI: animated Bootstrap progress bar, SSE reconnect on page reload mid-purge, automatic retry on SSE connection drop, disabled buttons during purge
• Handles TOCTOU race between controller check and service guard — InvalidOperationException caught silently without broadcasting misleading "failed" event
• Guard is always released in finally block, even on mid-purge failure or early return

Test plan

• All 1412 existing tests pass
• New PurgeAllCacheAsync_RejectsSecondConcurrentPurge — verifies only one purge runs, second throws
• New PurgeLRUCacheAsync_RejectsSecondConcurrentPurge — same for LRU variant
• New PurgeAllCacheAsync_BroadcastsProgressViaSse — verifies SSE events are broadcast with correct channel and payload structure
• New PurgeLRUCacheAsync_BroadcastsProgressViaSse — same for LRU variant
• New PurgeAllCacheAsync_GuardIsReleasedAfterFailure — verifies _purgeInProgress resets after completion
• Manual: navigate Admin Settings, click "Clear LRU Cache" → progress bar appears, updates, stats refresh on completion
• Manual: click purge button while another is running → "already in progress" warning
• Manual: refresh page during purge → progress bar reconnects automatically
• Manual: verify tile requests still work during purge (reduced lock contention)

[stef-k]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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