Implement base info aggregate endpoint

[FriggemannMichael]

Objective:
Implement GET /api/base-info/ for landing page statistics.

Acceptance criteria:

• Endpoint is public unless project rules later require authentication.
• Response includes review_count, average_rating, business_profile_count, and offer_count.
• average_rating is rounded to one decimal place.
• Empty data sets return stable numeric values instead of errors.

Frontend notes:
The frontend renders all keys automatically into base_info_* elements.

[FriggemannMichael]

TDD workflow for this project:

• Define the expected behavior before implementing the endpoint or model behavior.
• Write or outline the failing test first.
• Implement only enough code to make that test pass.
• Refactor after the test passes.
• Keep Django check, Ruff, and the relevant tests green from the local virtual environment.