Fall back system llm

[PrajanManojKumarRekha]

What this does

Adds a configurable LLM fallback system that keeps Groq as the primary provider and automatically routes to a secondary provider if Groq has an outage at the moment a background job fires.

Why

process_resume_task runs async via TaskIQ with no human in the loop. If Groq goes down when the task executes, the job fails silently and the candidate's resume never gets evaluated. Rate limiting is already handled but a provider outage had no recovery path until now.

Changes

• Added default_llm_provider and wired it through config and callers
• Added FallbackLLMProvider that wraps any two providers implementing LLMProviderInterface
• Groq stays primary, fallback is configurable via FALLBACK_LLM_MODEL_NAME and LITELLM_API_KEY
• Updated .env.example and docs with the new config fields

Testing

Verified end to end by forcing the primary to fail and confirming the fallback provider picks up the request and returns a valid response.