[FEAT] Add AI generation gateway endpoint in quiz_core

[MikeNovikoff]

Feature description

Expose a user-facing endpoint in the Go quiz_core service that accepts raw .txt study materials, proxies them to the internal llm-service, and returns generated cards. This endpoint acts as a gateway so the frontend never talks directly to the LLM service.

Scope

quiz_core/internal/api/handlers/llm/generate.go: Multipart form handler
quiz_core/internal/routes/routes.go: Register POST /core/quizzes/generate
quiz_core/internal/config/config.go: LLMService.URL from env (default http://llm-service:7000)

Acceptance Criteria

• Accepts multipart/form-data with document (.txt only) and form fields card_type, count
• Validates file size ≤ 1 MB and text length ≥ 10 characters
• Forwards request to llm-service via internal HTTP client with 120s timeout
• Returns 502 Bad Gateway with details if LLM service fails
• Returns 200 OK with parsed GenerateCardsResponse on success
• Properly closes file handles and uses io.LimitReader