fix(media): 统一图片结果 URL 化并限制同步 Base64

将上游 URL 直接持久化，内联媒体经对象存储后仅保留 URL 与内部定位元数据；异步轮询、任务详情和幂等重放统一使用零对象读取的 URL 投影，并增加 64KiB 响应门禁。

OpenAI 图片接口接受 url 与 b64_json，同步 Base64 限制为 20MiB 和每 Pod 2 并发；新增历史结果迁移清零门禁、结果指标和 API GOMEMLIMIT。

验证：API go test ./...、go vet、聚焦 race、pnpm openapi、pnpm lint/test/build、迁移安全检查与 docker compose config 均通过。

apps/api/internal/httpapi/compat_protocol_test.go

@@ -2,6 +2,7 @@ package httpapi
 
 import (
 	"context"
+	"encoding/base64"
 	"encoding/json"
 	"errors"
 	"net/http"
@@ -14,6 +15,55 @@ import (
 	"github.com/easyai/easyai-ai-gateway/apps/api/internal/store"
 )
 
+func TestSynchronousInlineResponseLimitAndCapacity(t *testing.T) {
+	request := map[string]any{"response_format": "b64_json"}
+	if !synchronousInlineResponseRequested("images.generations", request) {
+		t.Fatal("explicit b64_json image response was not detected")
+	}
+	encoded := strings.Repeat("A", base64.StdEncoding.EncodedLen(int(maxSynchronousInlineResponseBytes+1)))
+	if got := inlineMediaDecodedSize(map[string]any{"data": []any{map[string]any{"b64_json": encoded}}}); got <= maxSynchronousInlineResponseBytes {
+		t.Fatalf("decoded size=%d", got)
+	}
+	if got := base64DecodedSize(base64.StdEncoding.EncodeToString([]byte{1})); got != 1 {
+		t.Fatalf("padded Base64 decoded size=%d", got)
+	}
+
+	releaseFirst, err := acquireSynchronousInlineResponseSlot(t.Context(), "images.generations", request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer releaseFirst()
+	releaseSecond, err := acquireSynchronousInlineResponseSlot(t.Context(), "images.generations", request)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer releaseSecond()
+
+	cancelled, cancel := context.WithCancel(t.Context())
+	cancel()
+	_, err = acquireSynchronousInlineResponseSlot(cancelled, "images.generations", request)
+	if clients.ErrorCode(err) != "response_format_capacity_timeout" {
+		t.Fatalf("unexpected capacity error: %v", err)
+	}
+}
+
+func TestValidateMediaResponseFormat(t *testing.T) {
+	for _, value := range []string{"url", "b64_json", " B64_JSON "} {
+		body := map[string]any{"response_format": value}
+		if err := validateMediaResponseFormat("images.generations", body); err != nil {
+			t.Fatalf("validate %q: %v", value, err)
+		}
+	}
+	for _, value := range []any{"base64", 1, map[string]any{"type": "url"}} {
+		if err := validateMediaResponseFormat("images.generations", map[string]any{"response_format": value}); err == nil {
+			t.Fatalf("accepted invalid response_format: %#v", value)
+		}
+	}
+	if err := validateMediaResponseFormat("chat.completions", map[string]any{"response_format": map[string]any{"type": "json_object"}}); err != nil {
+		t.Fatalf("image validation affected chat response_format: %v", err)
+	}
+}
+
 func TestProtocolAPIKeyStoreFailureIs503InsteadOf401(t *testing.T) {
 	authenticator := auth.New("test-secret", "", "")
 	authenticator.LocalAPIKeyVerifier = func(context.Context, string) (*auth.User, error) {