feat: add API key load balancing for providers (#676)

Support multiple API keys per provider with random selection for load
balancing. When AI_MODELS_CONFIG has multiple apiKeyEnv values for
a provider, requests will randomly select one available key.

- Update schema to accept apiKeyEnv as string or string array
- Add random key selection in resolveApiKey()
- Update validation to check at least one key exists
- Add tests for array format support

lib/types/model-config.ts

@@ -73,8 +73,9 @@ export interface FlattenedModel {
     source?: "user" | "server"
     // Whether this model is the server default (matches AI_MODEL env var)
     isDefault?: boolean
-    // Custom env var names for server models (allows multiple API keys per provider)
-    apiKeyEnv?: string
+    // Custom env var name(s) for server models
+    // Can be a single string or array of strings for load balancing
+    apiKeyEnv?: string | string[]
     baseUrlEnv?: string
 }