RAG: Knowledge Base API endpoints

[sre-helmcode]

Description

Create handlers_knowledge.go with:

• GET /api/knowledge/status — Qdrant status, doc counts, embedding model
• GET /api/knowledge/documents — List org documents
• POST /api/knowledge/documents — Upload (multipart, max 50MB)
• GET /api/knowledge/documents/:id — Get document
• DELETE /api/knowledge/documents/:id — Delete doc + Qdrant chunks

Upload flow: validate → save to disk → create DB record → return 201 → async process (ensure Qdrant + Ollama + pull nomic-embed-text → parse → chunk → embed → upsert)

Acceptance Criteria

• All 5 endpoints working
• File validation (extension whitelist, MIME type, 50MB limit)
• Async processing with status updates
• Delete cleans up Qdrant chunks + disk file
• Integration tests