Fix cold-cache tile loading: 503 + client-side retry for budget exhaustion

[stef-k]

Summary

• Fix cold-cache tile loading: budget-exhausted tiles now return HTTP 503 + Retry-After instead of 404, so the client can retry instead of showing permanent gray areas
• Add RetryTileLayer (fetch-based Leaflet TileLayer subclass) that detects 503, reads Retry-After, and retries with exponential backoff -- tiles progressively stream in
• Centralize tile layer creation via createTileLayer() factory, replacing duplicated boilerplate across 13 JS files
• Add RequestIdLoggingMiddleware + Serilog FromLogContext enrichment -- every log line now includes RequestId for cross-request correlation
• Fix ghost metadata: prevent DB rows with Size=0 when tile fetch is aborted by budget exhaustion
• Extract DbMetadataZoomThreshold constant replacing magic number 9 (12 occurrences)

Closes #206

Test plan

• dotnet build passes (0 errors)
• dotnet test passes (1404 tests)
• Cold-cache test: clear tile cache, load map at zoom 18 -- tiles should progressively fill in over ~10s instead of showing persistent gray areas
• Verify server logs show 503 (not 404) for budget-exhausted tiles, and each log line includes RequestId in {Properties:j}
• Verify SELECT * FROM "TileCacheMetadata" WHERE "Size" = 0 returns zero rows after cold-cache load
• Code review via code-reviewer subagent