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/services/ai_policy_service.py

@@ -0,0 +1,77 @@
+"""Per-tree AI model policy — owner-only. Assigns which configured provider
+members and the recommender use; the owner may use any configured provider.
+
+The operator decides which providers exist (env / registry); the tree owner
+decides who uses which. See app/api/deps.py for the registry.
+"""
+
+import uuid
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.api.deps import configured_llm_providers
+from app.models.enums import MembershipRole
+from app.models.tree import Tree
+from app.models.user import User
+from app.services import privacy
+from app.services.exceptions import Forbidden
+
+
+async def _require_owner(session: AsyncSession, *, actor: User, tree: Tree) -> None:
+    role = await privacy.get_membership_role(session, actor.id, tree.id)
+    if role is not MembershipRole.owner:
+        raise Forbidden("only the tree owner can configure AI")
+
+
+def _names() -> set[str]:
+    return {p["name"] for p in configured_llm_providers()}
+
+
+async def get_policy(session: AsyncSession, *, actor: User, tree: Tree) -> dict:
+    await _require_owner(session, actor=actor, tree=tree)
+    from app.core.config import get_settings
+
+    return {
+        "member_provider": tree.ai_member_provider,
+        "recommender_provider": tree.ai_recommender_provider,
+        "configured_providers": configured_llm_providers(),
+        "default_provider": get_settings().default_llm_provider,
+    }
+
+
+async def update_policy(
+    session: AsyncSession,
+    *,
+    actor: User,
+    tree: Tree,
+    member_provider: str | None,
+    recommender_provider: str | None,
+) -> dict:
+    await _require_owner(session, actor=actor, tree=tree)
+    valid = _names()
+    for value in (member_provider, recommender_provider):
+        if value is not None and value not in valid:
+            raise Forbidden(f"'{value}' is not a configured provider")
+    tree.ai_member_provider = member_provider
+    tree.ai_recommender_provider = recommender_provider
+    await session.commit()
+    await session.refresh(tree)
+    return await get_policy(session, actor=actor, tree=tree)
+
+
+# --- Resolution helpers (for the future assistant / recommender) -------------
+
+def provider_name_for_member(tree: Tree) -> str | None:
+    """Provider an ordinary member's assistant should use, if any."""
+    return tree.ai_member_provider
+
+
+def provider_name_for_recommender(tree: Tree) -> str | None:
+    return tree.ai_recommender_provider
+
+
+def provider_name_for_owner(tree: Tree, requested: str | None = None) -> str | None:
+    """The owner may use any configured provider; default to the requested one."""
+    if requested and requested in _names():
+        return requested
+    return tree.ai_member_provider  # fall back to the member model