ensure_node|edge_attr_index for SQLGraph

[yfukai]

This pull request introduces a new feature for the SQLGraph backend: the ability to create explicit database indexes on node and edge attributes to improve query performance, especially for frequently filtered attributes. The documentation and tests have been updated to reflect and validate this functionality.

SQLGraph indexing improvements:

• Added methods ensure_node_attr_index and ensure_edge_attr_index to SQLGraph for creating indexes on node and edge attribute columns, including support for composite and unique indexes. (src/tracksdata/graph/_sql_graph.py)
• Updated documentation to describe the new index feature and provide usage examples for creating indexes on attributes. (docs/concepts.md)
• Updated the project README to mention SQLGraph's ability to index frequently queried attributes for faster filtering. (README.md)

Testing and validation:

• Added tests to ensure index creation works as expected, including checks for composite and unique indexes, and error handling for missing columns. (src/tracksdata/graph/_test/test_graph_backends.py)
• Added sqlalchemy import to support index inspection in tests. (src/tracksdata/graph/_test/test_graph_backends.py)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.45%. Comparing base (c5af9f3) to head (3594699).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tracksdata/graph/_sql_graph.py	83.33%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #223      +/-   ##
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- Coverage   88.45%   88.45%   -0.01%     
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  Files          55       55              
  Lines        3890     3976      +86     
  Branches      674      694      +20     
==========================================
+ Hits         3441     3517      +76     
- Misses        267      275       +8     
- Partials      182      184       +2     

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[yfukai]

Maybe we need drop functions

[JoOkuma]

@yfukai, awesome PR, I had hadn't thought of adding this.

[JoOkuma]

@yfukai this is awesome. Could you briefly mention what kind of speed-up we can expect with this? 2x, 10x?