Refactor chatbot to use dependency injection for corpus building

[Alen983]

The chatbot's _build_corpus() function is tightly coupled and hard to test.
Refactoring improves testability and extensibility.

Relevant part of the codebase:

• backend/app/routers/chatbot.py

Expected outcome:

• Extract corpus building into a service class
• Use dependency injection for database access
• Make it easier to add new data sources (e.g., external APIs)
• Add unit tests for corpus building logic