feat(providers): route only the generate submit through the proxy

Extends the adobe split to leonardo, krea, imagine, runway (image+video) and chatgpt: reference-image upload, polling and result download egress on the local IP; only the generate/task-create submit uses proxy.url. chatgpt keeps its anti-bot-guarded submit+upload phase on the proxy and moves only the existing second (poll/resolve/download) session to local. custom is already direct. Build+vet pass; adobe verified live, the other providers are code-complete but not yet live-tested (no dev accounts).

backend/internal/provider/imagine/image.go

@@ -101,7 +101,7 @@ func (c *Client) pollImage(ctx context.Context, token, userID, batchID string) (
 
 	url := teamsBase + "/v1/org/" + userID + "/objects?batch=true&limit=50&service=image,chat-image"
 	for {
-		body, status, err := c.apiGet(ctx, token, url)
+		body, status, err := c.apiGetP(ctx, token, url, false)
 		if err == nil && status == 200 {
 			var resp struct {
 				Data []struct {
@@ -181,7 +181,7 @@ func firstImageURL(raw string) string {
 }
 
 func (c *Client) download(ctx context.Context, url string) ([]byte, error) {
-	client, err := c.newTLSClient()
+	client, err := c.newDirectTLSClient()
 	if err != nil {
 		return nil, err
 	}