Improve dependencies management for users and testing

[FlxPo]

Motivation

• Make CI reproducible by pinning tested dependency sets instead of resolving floating versions on every run (to avoid errors such as the one encountered in External mobility surveys #274, where a polars release generated an error unrelated to the PR).
• Add an explicit minimum-dependency compatibility check without slowing down PR validation too much.

Changes

• Added version bounds to Python dependencies in pyproject.toml.
• Added pinned and minimum dependency snapshots in requirements-ci.txt and requirements-min.txt (generated with uv, see the first line of each file).
• Refactored CI into a reusable workflow in test-reusable.yml that can run with different requirements.
• Added a minimum-dependency workflow in install-and-test-min.yml that runs only on pushes to main.
• Updated environment-test.yml so micromamba manages only Python, uv, and osmium-tool.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.92%. Comparing base (d7e75e6) to head (241f8ae).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #275      +/-   ##
==========================================
+ Coverage   58.90%   58.92%   +0.01%     
==========================================
  Files         128      128              
  Lines        6712     6715       +3     
==========================================
+ Hits         3954     3957       +3     
  Misses       2758     2758              

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