Implement pre-computed etag support for mbtiles sources

[Copilot]

This PR implements more performant etag support by allowing mbtiles backends to supply pre-computed etags when available, eliminating unnecessary CPU usage from computing xxhash over tile data.

Background

CDNs rely heavily on etags (usually a hash) to identify if tiles have changed. Many .mbtiles files already store tiles with pre-computed hashes using two tables - one stores z, x, y, hash columns, and another table stores hash, data blob. Currently, Martin computes etags by running a non-cryptographical hash function over the output even when pre-computed hashes are available, causing unnecessary CPU usage.

Changes Made

1. Enhanced Source Trait

• Added get_tile_etag() method with default implementation returning None
• Added Sync bound for thread safety
• Maintains full backward compatibility

2. Optimized MbtSource Implementation

• Detects and stores MbtType during initialization
• Implements get_tile_etag() to return pre-computed hashes for supported types:
  • MbtType::FlatWithHash: Uses hash from tiles_with_hash table
  • MbtType::Normalized: Uses hash from images table tile_id
  • MbtType::Flat: Returns None (falls back to computed hash)

3. Updated Tile Serving Pipeline

• Extended Tile struct with optional etag field
• Modified tile serving logic to use pre-computed etag for single-source requests
• Updated compression/decompression functions to preserve etag
• Falls back to xxhash computation when no pre-computed etag is available

4. HTTP Response Integration

• Modified get_http_response() to prefer pre-computed etag over computed hash
• Maintains identical HTTP behavior - completely transparent to clients

Performance Impact

• For mbtiles with pre-computed hashes: Eliminates xxhash computation entirely - instant etag lookup
• For flat mbtiles or other sources: No performance change - same xxhash computation as before
• Memory overhead: Minimal - only adds optional String to Tile struct
• Network behavior: Identical - etag headers work exactly the same

Testing

Added comprehensive tests verifying:

• Pre-computed etag functionality with real mbtiles files
• HTTP responses correctly use pre-computed etags
• Backward compatibility - all existing tests continue to pass
• Sources without pre-computed etags work normally

Backward Compatibility

✅ Fully backward compatible - no breaking changes

• Existing sources (PostgreSQL, PMTiles, etc.) continue working exactly as before
• Flat mbtiles files continue working with computed etags
• All existing functionality preserved

Example Usage

// For FlatWithHash or Normalized mbtiles:
let etag = source.get_tile_etag(xyz, None).await?;
// Returns Some("80EE46337AC006B6BD14B4FA4D6E2EF9") - pre-computed hash

// For Flat mbtiles or other sources:
let etag = source.get_tile_etag(xyz, None).await?;
// Returns None - will compute xxhash as before

The implementation successfully addresses the performance bottleneck described in the issue while maintaining full compatibility with existing functionality.

Fixes #1917.

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[CommanderStorm]

For what this PR is worth, I think this is simpler if re-done by a human.

[nyurik]

I agree that this should be done by a human. The goal was to see how this system works in general, and if we can use it to solve simpler tasks

[CommanderStorm]

It definitively can - just in rust the quality of code it produces is not great.

In react though..