Per-tree AI model policy (owner-only admin view)

The operator decides which model providers exist (env / registry — Anthropic,
OpenAI, x.AI, Ollama, several at once). The *tree owner* decides who uses which:

- Members' assistant -> one configured provider (or none)
- Recommender (association/connection finder) -> one configured provider (or none)
- Owner -> may use any configured provider

Backend: two nullable columns on `trees` (ai_member_provider,
ai_recommender_provider) + migration; `configured_llm_providers()` exposes the
registry as {name, model} with no secrets; owner-gated GET/PATCH
/trees/{id}/ai validate names against the configured set. Frontend: owner-only
"AI models" page with a dropdown per role, graceful 403 for non-owners, and a
sidebar link.

Per-model-within-a-provider selection is a follow-up; today each provider maps
to its single configured model.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/api/v1/__init__.py

@@ -3,6 +3,7 @@
 from fastapi import APIRouter
 
 from app.api.v1 import (
+    ai,
     auth,
     citations,
     cleanup,
@@ -36,3 +37,4 @@ api_router.include_router(cleanup.router)
 api_router.include_router(public.router)
 api_router.include_router(members.router)
 api_router.include_router(proposals.router)
+api_router.include_router(ai.router)