#261- Replace manual type checking with PEP 484 type hints

[Cubix33]

Closes #261

📝 Description

Removes the outdated and clunky type_check_all() method in favor of modern Python type hinting. This cleans up the codebase, removes boilerplate validation logic, and allows linters and IDEs to catch type errors automatically.

🛠️ Changes Made

• Deleted the manual type_check_all() method in src/llm.py.
• Added PEP 484 type annotations (e.g., : str, -> dict) to __init__, build_prompt, main_loop, add_response_to_json, handle_plural_values, and get_data.
• Cleaned up remaining inline comments and docstrings for a leaner, more readable file.

🧪 How to Test

1. Run the application (e.g., make exec or docker compose exec app python3 src/main.py).
2. Verify the LLM extraction and PDF filling complete successfully without throwing any type errors.