feat: add per-tier fallback field to model config schema

Each model tier (fast, default, complex, local) can now specify an
optional fallback provider config that the router will try before
falling through to the global fallback chain.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/config/schema.ts

@@ -18,7 +18,7 @@ const serverSchema = z.object({
   auth_http: z.boolean().default(true),
 });
 
-const modelConfigSchema = z.object({
+const modelConfigBaseSchema = z.object({
   provider: z.enum(['anthropic', 'openai', 'gemini', 'ollama', 'llamacpp', 'openrouter', 'bedrock', 'github']),
   model: z.string(),
   endpoint: z.string().optional(),
@@ -29,6 +29,10 @@ const modelConfigSchema = z.object({
   context_window: z.number().optional(),
 });
 
+const modelConfigSchema = modelConfigBaseSchema.extend({
+  fallback: modelConfigBaseSchema.optional(),
+});
+
 const modelsSchema = z.object({
   local: modelConfigSchema.optional(),
   fast: modelConfigSchema.optional(),