feat: add guarded image generation controls

asset-gateway.mjs

@@ -32,10 +32,10 @@ export const ASSET_PLUGIN_CATALOG = [
 ];
 
 export const IMAGE_MAKE_PURPOSE_CATALOG = [
-  { id: 'inline_image', label: '页面正文图片', presetId: 'memind_square_illustration' },
-  { id: 'hero', label: '页面头图', presetId: 'memind_dark_hero' },
-  { id: 'card_cover', label: '卡片封面', presetId: 'memind_card_cover' },
-  { id: 'feed_cover', label: '信息流与缩略图源图', presetId: 'memind_feed_cover_source' },
+  { id: 'inline_image', label: '页面正文图片', presetId: 'memind_square_illustration', strategy: 'generate', sourcePurpose: null },
+  { id: 'hero', label: '页面头图', presetId: 'memind_dark_hero', strategy: 'generate', sourcePurpose: null },
+  { id: 'card_cover', label: '卡片封面图', presetId: 'memind_card_cover', strategy: 'derive', sourcePurpose: 'hero' },
+  { id: 'feed_cover', label: '信息流缩略图', presetId: 'memind_feed_cover_source', strategy: 'derive', sourcePurpose: 'hero' },
 ];
 
 const imageMakePurposeIds = new Set(IMAGE_MAKE_PURPOSE_CATALOG.map((purpose) => purpose.id));
@@ -281,10 +281,21 @@ export function createAssetGatewayConfigService(pool, { llmProviderService = nul
       if (!resolved.plugin.purposes?.[purpose]) {
         return { ok: false, code: 'purpose_disabled', message: '该图片用途未启用，可安全降级到原有流程' };
       }
+      const purposeConfig = IMAGE_MAKE_PURPOSE_CATALOG.find((item) => item.id === purpose);
+      if (purposeConfig?.strategy === 'derive' && resolved.plugin.purposes?.hero) {
+        return {
+          ok: false,
+          code: 'reuse_hero_required',
+          message: '页面头图已启用：请只生成一次 hero，并从该主图派生卡片封面和信息流缩略图',
+          sourcePurpose: 'hero',
+        };
+      }
       return {
         ok: true,
         purpose,
-        presetId: IMAGE_MAKE_PURPOSE_CATALOG.find((item) => item.id === purpose)?.presetId,
+        presetId: purposeConfig?.presetId,
+        strategy: purposeConfig?.strategy ?? 'generate',
+        sourcePurpose: purposeConfig?.sourcePurpose ?? null,
         plugin: resolved.plugin,
       };
     },