refactor: Simplify topic extraction logic in TopicService by removing Pydantic schema, enhancing JSON response handling, and adding fuzzy matching for improved topic validation.

feat: Introduce predefined topics list in TopicService and enhance topic extraction logic to validate against this list, ensuring accurate topic matching and improved error handling.

fix: Update topic extraction instructions in TopicService to specify that the main idea should be captured in simple English.

fix: Clarify topic extraction instructions in the TopicService to specify that the output should be a single sentence capturing the main idea.

feat: Add topic-related schemas and API routes for topic management, along with LangChain dependencies in requirements.