feat: add admin controls for LLM provider visibility and default model

[Nithishvb]

Summary

Closes #1183— gives admins control over which LLM providers/models users can see and which model is selected by default.

Problem

On self-hosted AWS instances, Bedrock is auto-generated from environment credentials and could not be removed. Deleting it from the UI didn't persist — it reappeared on the next request. There was no way to hide a provider or force a default model. The default shown to new users was purely positional (first available model).

Solution

Two-layer configuration (env vars + DB) that work together:

Deactivate providers

• Env var: NAO_DISABLED_PROVIDERS=bedrock,vertex (comma-separated, always applied)
• Admin UI: Toggle badges in Settings → Models section
• Effective: Union of env + DB lists (env can add constraints, admin can add more)

Choose default model

• Env var: NAO_DEFAULT_MODEL=anthropic:claude-sonnet-4-6 (provider:modelId format, fallback when DB is unset)
• Admin UI: Dropdown in Settings → Models section
• Effective: DB value takes precedence over env (admin can always override)

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