Fix: Correctly serialize JSON for database updates

Created by: Edouard-Legoupil

This commit fixes a TypeError that occurred when saving proposal sections to the database. The error was caused by passing a Python dictionary to the database driver instead of a JSON string.

The process_section function in backend/api/proposals.py has been refactored to ensure the generated_sections dictionary is always serialized to a JSON string using json.dumps() before being passed to the database.

Additionally, this commit includes several improvements to the codebase:

• The database update logic in process_section has been centralized to handle both new and regenerated content consistently.

• Redundant database update logic has been removed from backend/utils/proposal_logic.py.

• The test suite has been significantly overhauled to use a real in-memory SQLite database instead of mocks, providing more reliable and realistic testing.

• All relevant tests have been updated to use the new test database and an authenticated client fixture.

• Added pytest-mock to the testing dependencies.

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