Add forms route tests and register AppError handler

[Mingqwqqaq]

Summary

This PR improves route-level reliability around form submission. Closes #265

Changes

• register the custom AppError handler in api/main.py
• fetch the template only once in /forms/fill
• add route tests for:
  • successful /forms/fill
  • missing-template /forms/fill returning 404
• strengthen /templates/create assertions

Why

The forms route already raises AppError for missing templates, but the app was not registering the exception handler, so the intended API behavior was incomplete.

The existing tests/test_forms.py file was also still a stub, so the main submission path had no meaningful coverage.

Testing

Added API tests using monkeypatch to isolate route behavior from:

• PDF processing
• Ollama/LLM calls
• file generation side effects