feat: add Renovate, expand test matrix with dependency floors

[marcgibbons]

• Add renovate.json: weekly schedule, automerge dev deps, pydantic/ pydantic-settings/django pinned to minimum supported versions (2.7., 2.13., 5.2.*), testing tools and pre-commit hooks grouped
• Add .github/workflows/renovate.yml: self-hosted via lincolnloop GitHub App
• Add renovatebot/pre-commit-hooks to .pre-commit-config.yaml
• Expand test matrix: include job for Python 3.10 + pydantic 2.7 + pydantic-settings 2.13 + django 5.2 to test minimum supported config
• Bump django dev dep floor from >=3.2 to >=5.2

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.42%. Comparing base (235cc28) to head (9436167).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #61   +/-   ##
=======================================
  Coverage   98.42%   98.42%           
=======================================
  Files           8        8           
  Lines         380      380           
  Branches        5        5           
=======================================
  Hits          374      374           
  Misses          3        3           
  Partials        3        3           

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