PHASE2-S2: Celery + Redis async workers & rate limiting

[psypherion]

Setup async job queue & caching layer for scaling

• Install & configure Celery + Redis
• Batch embedding jobs (100s at once)
• Async ingestion pipeline (don't block API)
• Nightly clustering/topic refresh jobs
• Per-user rate limiting (10 req/min free tier)
• API key validation + usage tracking