Server-side AI Q&A with mandatory retrieval + citations.

Body: {ticker: str, question: str, conversation_id?: str, request_id?: str, context?: {kind: "bdc", ticker?: str, borrower?: str, entry?: str}} context (SF-PRIVATE-CREDIT-COPILOT T3): the BDC / private-credit view the user is on. Validated by services/ai_copilot_grounding.parse_copilot_context (400 on a bad kind / ticker / borrower); effective only when PLATFORM_AI_PRIVATE_CREDIT is on. Returns:

• JSON (advisory refusal or error), or
• SSE stream of LLM tokens terminated by a final data: {"citations":[...], "grounded": bool, "grounding": ...}\n\n then data: [DONE]\n\n.