fix(chat): add saved AI defaults and harden suggestions

.memory/research/litellm-zen-provider-catalog.md

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+# Research: LiteLLM provider catalog and OpenCode Zen support
+
+Date: 2026-03-08
+Related plan: [AI travel agent in Collections Recommendations](../plans/ai-travel-agent-collections-integration.md)
+
+## LiteLLM provider enumeration
+- Runtime provider list is available via `litellm.provider_list` and currently returns 128 provider IDs in this environment.
+- The enum source `LlmProviders` can be used for canonical provider identifiers.
+
+## OpenCode Zen compatibility
+- OpenCode Zen is **not** a native LiteLLM provider alias.
+- Zen can be supported via LiteLLM's OpenAI-compatible routing using:
+  - provider id in app: `opencode_zen`
+  - model namespace: `openai/<zen-model>`
+  - `api_base`: `https://opencode.ai/zen/v1`
+- No new SDK dependency required.
+
+## Recommended backend contract
+- Add backend source-of-truth endpoint: `GET /api/chat/providers/`.
+- Response fields:
+  - `id`
+  - `label`
+  - `available_for_chat`
+  - `needs_api_key`
+  - `default_model`
+  - `api_base`
+- Return all LiteLLM runtime providers; mark non-mapped providers `available_for_chat=false` for display-only compliance.
+
+## Data/storage compatibility notes
+- Existing `UserAPIKey(provider)` model supports adding `opencode_zen` without migration.
+- Consistent provider ID usage across serializer validation, key lookup, and chat request payload is required.
+
+## Risks
+- Zen model names may evolve; keep default model configurable in backend mapping.
+- Full provider list is large; UI should communicate unavailable-for-chat providers clearly.