fix(media): 支持大尺寸图像响应并补齐迁移契约

- 为图像协议放宽 JSON 响应上限并显式处理读取与超限错误
- 合并基础模型与平台能力，避免运行时丢失分辨率和比例约束
- 固化稳定 Gemini 图像模型的能力、计价和 preview 兼容别名

apps/api/internal/store/candidates.go

@@ -43,12 +43,12 @@ func (s *Store) ListModelCandidates(ctx context.Context, model string, modelType
        CASE
          WHEN b.capabilities #> '{text_generate,supportedApiProtocols}' IS NOT NULL
          THEN jsonb_set(
-           COALESCE(m.capabilities, '{}'::jsonb),
+           COALESCE(b.capabilities, '{}'::jsonb) || COALESCE(m.capabilities, '{}'::jsonb),
            '{text_generate,supportedApiProtocols}',
            b.capabilities #> '{text_generate,supportedApiProtocols}',
            true
          )
-         ELSE m.capabilities
+         ELSE COALESCE(b.capabilities, '{}'::jsonb) || COALESCE(m.capabilities, '{}'::jsonb)
        END AS effective_capabilities,
        m.capability_override,
        COALESCE(b.base_billing_config, '{}'::jsonb), m.billing_config, m.billing_config_override,