fix: correct background_source enum values for Kling 3.0 Motion Control

Per Kie docs, background_source accepts "input_video"/"input_image",
not "image"/"video". The previous wrong values were being sent to the
API after schema defaults pre-population, causing request rejection.

Also added background_source to getKieModelDefaults() for consistency.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/app/api/generate/providers/kie.ts

@@ -139,6 +139,7 @@ export function getKieModelDefaults(modelId: string): Record<string, unknown> {
       return {
         mode: "pro",
         character_orientation: "video",
+        background_source: "input_video",
       };
 
     // Kling 2.6 video models

src/app/api/models/[modelId]/route.ts

@@ -990,7 +990,7 @@ function getKieSchema(modelId: string): ExtractedSchema {
       parameters: [
         { name: "mode", type: "string", description: "Generation mode", enum: ["std", "pro"], default: "pro" },
         { name: "character_orientation", type: "string", description: "Character orientation source", enum: ["image", "video"], default: "video" },
-        { name: "background_source", type: "string", description: "Background source", enum: ["image", "video"], default: "video" },
+        { name: "background_source", type: "string", description: "Background source", enum: ["input_video", "input_image"], default: "input_video" },
       ],
       inputs: [
         { name: "prompt", type: "text", required: false, label: "Prompt" },