Add BARE setting for minimal database-free apps

[SmileyChris]

Summary

• Add BARE=True setting for lightweight apps that don't need persistent storage or authentication
• Make STATICFILES_DIRS conditional on the directory existing

BARE mode features

When BARE=True is set:

• Cookie-based sessions (signed_cookies backend)
• Cookie-based messages (CookieStorage)
• In-memory SQLite database by default
• No auth, admin, or contenttypes apps installed
• Middleware and context processors adapt to installed apps
• Skips migrations and superuser prompts on startup

Example usage

from nanodjango import Django

app = Django(BARE=True)

@app.route("/")
def index(request):
    count = request.session.get("count", 0) + 1
    request.session["count"] = count
    return f"Visit count: {count}"

Test plan

• All existing tests pass (90/91, 1 pre-existing failure unrelated to this PR)
• New tests for BARE mode settings verification
• New tests for cookie-based sessions and messages
• New test for STATICFILES_DIRS behavior

[codecov]

Codecov Report

❌ Patch coverage is 59.45946% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.00%. Comparing base (e91c6e3) to head (0497e01).

Files with missing lines	Patch %	Lines
nanodjango/app.py	0.00%	12 Missing ⚠️
nanodjango/settings.py	88.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   47.79%   48.00%   +0.21%     
==========================================
  Files          26       26              
  Lines        1904     1931      +27     
==========================================
+ Hits          910      927      +17     
- Misses        994     1004      +10     

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