fix: add api_key field to provider schema for inline key setup (#57)

The provider creation form only collected api_key_env (the env var
name) but not the actual key value. New providers were always created
as "unconfigured" because no key was stored.

Add an optional secret api_key field so the dashboard can pass the
key value during creation. The backend strips it from the TOML and
saves it to secrets.env instead.

schema.toml

@@ -41,6 +41,12 @@ required = true
 description = "Whether an API key is needed (false for local providers)"
 example = true
 
+[provider.fields.api_key]
+type = "string"
+required = false
+description = "API key value (stored securely in secrets.env, not in the TOML file)"
+secret = true
+
 # --- Provider: nested [[models]] section ---
 
 [provider.sections.models]