Fixing successors large query

[JoOkuma]

Fixes #249
cc: @yfukai

We might consider doing the query differently in the future to improve the performance.
There are a lot of notes in https://docs.pola.rs/api/python/stable/reference/api/polars.read_database.html#polars.read_database

[JoOkuma]

I'm waiting for the slow test to pass. Can you check if it works on your end, @yfukai ?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.33%. Comparing base (574694d) to head (c63c503).

Files with missing lines	Patch %	Lines
src/tracksdata/graph/_sql_graph.py	90.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #250      +/-   ##
==========================================
- Coverage   88.35%   88.33%   -0.02%     
==========================================
  Files          56       56              
  Lines        4104     4116      +12     
  Branches      714      716       +2     
==========================================
+ Hits         3626     3636      +10     
- Misses        291      292       +1     
- Partials      187      188       +1     

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