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/deps.py

@@ -101,6 +101,22 @@ def build_llm_providers() -> dict[str, LLMProvider]:
     return providers
 
 
+def configured_llm_providers() -> list[dict]:
+    """Configured LLM providers as {name, model} — for the AI admin view (no
+    secrets). Mirrors build_llm_providers() without constructing clients."""
+    s = get_settings()
+    out: list[dict] = []
+    if s.anthropic_api_key:
+        out.append({"name": "anthropic", "model": s.anthropic_model})
+    if s.openai_api_key:
+        out.append({"name": "openai", "model": s.openai_model})
+    if s.xai_api_key:
+        out.append({"name": "xai", "model": s.xai_model})
+    if s.ollama_enabled:
+        out.append({"name": "ollama", "model": s.ollama_model})
+    return out
+
+
 def get_llm_provider(name: str | None = None) -> LLMProvider:
     """The named LLM provider, or the configured default, or Null if unconfigured."""
     providers = build_llm_providers()