Add settings callback support

[SmileyChris]

Summary

• Allow using callbacks to modify default settings rather than replacing them
• If a setting value is callable and the setting already exists in nanodjango's defaults, call it with the current value and use the return value
• Add run_app_code test helper for creating and running temporary nanodjango apps

Example:

app = Django(
    MIDDLEWARE=lambda m: [MyPreMiddleware] + m + [MyPostMiddleware],
    INSTALLED_APPS=lambda apps: [a for a in apps if "admin" not in a],
)

Closes #74

Test plan

• Test callback modifies MIDDLEWARE
• Test callback filters INSTALLED_APPS
• Test literal callable for new settings (like WHITENOISE_ADD_HEADERS_FUNCTION) is kept as-is
• Test non-callable values work normally

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.96%. Comparing base (e91c6e3) to head (ecfe69f).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
nanodjango/app.py	0.00%	2 Missing ⚠️
nanodjango/testing/utils.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   47.79%   47.96%   +0.17%     
==========================================
  Files          26       26              
  Lines        1904     1916      +12     
==========================================
+ Hits          910      919       +9     
- Misses        994      997       +3     

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[brianhelba]

@radiac This is a very helpful feature. I've been waiting in anticipation for it.

Could you create a new v0.14 release to make this easily usable downstream?

[radiac]

I was just finishing adding another feature - 0.14 is up now.