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/models/tree.py

@@ -36,6 +36,11 @@ class Tree(Base, UUIDPrimaryKey, Timestamps, SoftDelete):
             use_alter=True,
         )
     )
+    # Per-tree AI model policy (owner-configured). The names reference configured
+    # providers from the registry; null = that role has no model. The owner may
+    # use any configured provider; these limit members + the recommender.
+    ai_member_provider: Mapped[str | None] = mapped_column(String(32))
+    ai_recommender_provider: Mapped[str | None] = mapped_column(String(32))
 
 
 class TreeMembership(Base, UUIDPrimaryKey, Timestamps):