feat: add Grok (grok.com) video provider; runway nano-banana image; unify runway/grok pool policy

- grok provider: imagine text/image-to-video (media.post.create → conversations/new),
  GetGrokCreditsConfig credit query (remaining = 100 - used) + weekly reset, spoofed
  x-statsig-id (no Cloudflare clearance needed), /api/auth/session email lookup,
  6 reference images, 10 concurrent jobs/account, no token refresh (dead = dead)
- runway: nano-banana-2 image flow (Nano Banana 2); drop pre-deduct + post-success
  reconcile; out-of-credits/403 → dead (no revive); 10-ratio support
- imagine: drop post-success credit reconcile (consistent with krea)
- account gate: per-account N-concurrency (grok=10, others=1)
- admin: provider health lists all 7 providers; frontend import auto-detects Grok SSO
- docs: README (CN/EN) updated to 7 providers + Grok

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/provider/runway/image.go

@@ -0,0 +1,139 @@
+package runway
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"image"
+	"strings"
+	"time"
+
+	http "github.com/bogdanfinn/fhttp"
+	tlsclient "github.com/bogdanfinn/tls-client"
+	"github.com/google/uuid"
+)
+
+// GenerateImage runs the Runway "Nano Banana 2" (gemini_3_1_flash_image)
+// text/image-to-image pipeline: upload each reference image (DATASET +
+// DATASET_PREVIEW → dataset) to obtain its {assetId, url}, create a gemini image
+// task and poll it to completion, then download the rendered PNG. teamID is the
+// workspace id; if empty it's derived from the token. aspectRatio is passed
+// through as-is (e.g. "16:9"); imageSize is the "1K"/"2K"/"4K" tier. refs may be
+// empty (pure text-to-image).
+func (c *Client) GenerateImage(ctx context.Context, token, teamID, prompt, aspectRatio, imageSize string, refs [][]byte) ([]byte, map[string]any, error) {
+	token = strings.TrimSpace(strings.TrimPrefix(token, "Bearer "))
+	if token == "" {
+		return nil, nil, ErrAuth
+	}
+	if teamID == "" {
+		teamID = TeamIDFromToken(token)
+	}
+	if teamID == "" {
+		return nil, nil, errors.New("runway: no team id")
+	}
+	if strings.TrimSpace(aspectRatio) == "" {
+		aspectRatio = "16:9"
+	}
+	if strings.TrimSpace(imageSize) == "" {
+		imageSize = "1K"
+	}
+
+	client, err := c.newTLSClient()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	var refImages []map[string]any
+	for i, raw := range refs {
+		if len(raw) == 0 {
+			continue
+		}
+		filename := fmt.Sprintf("ref_%s_%d.png", time.Now().UTC().Format("20060102_150405"), i+1)
+		assetID, url, upErr := c.uploadReference(ctx, client, token, teamID, filename, raw)
+		if upErr != nil {
+			return nil, nil, upErr
+		}
+		refImages = append(refImages, map[string]any{
+			"tag":     fmt.Sprintf("IMG_%d", i+1),
+			"assetId": assetID,
+			"url":     url,
+		})
+	}
+
+	taskID, err := c.createImageTask(ctx, client, token, teamID, prompt, aspectRatio, imageSize, refImages)
+	if err != nil {
+		return nil, nil, err
+	}
+	artifactURL, err := c.pollTask(ctx, client, token, teamID, taskID)
+	if err != nil {
+		return nil, nil, err
+	}
+	data, err := c.download(ctx, client, artifactURL)
+	if err != nil {
+		return nil, nil, err
+	}
+	meta := map[string]any{
+		"provider":  "runway",
+		"task_id":   taskID,
+		"team_id":   teamID,
+		"image_url": artifactURL,
+	}
+	return data, meta, nil
+}
+
+// uploadReference uploads one reference image through the dataset pipeline
+// (DATASET_PREVIEW + DATASET uploads → /v1/datasets) and returns its asset id
+// (= dataset id) and the cloudfront URL the task references.
+func (c *Client) uploadReference(ctx context.Context, client tlsclient.HttpClient, token, teamID, filename string, data []byte) (string, string, error) {
+	cfg, _, err := image.DecodeConfig(bytes.NewReader(data))
+	if err != nil {
+		return "", "", errors.New("runway: failed to decode reference image")
+	}
+	previewUploadID, _, err := c.uploadFile(ctx, client, token, teamID, filename, "DATASET_PREVIEW", data)
+	if err != nil {
+		return "", "", err
+	}
+	// The DATASET upload's completed URL is exactly what the task references.
+	datasetUploadID, refURL, err := c.uploadFile(ctx, client, token, teamID, filename, "DATASET", data)
+	if err != nil {
+		return "", "", err
+	}
+	assetID, _, err := c.createDataset(ctx, client, token, teamID, filename, datasetUploadID, previewUploadID, cfg.Width, cfg.Height)
+	if err != nil {
+		return "", "", err
+	}
+	return assetID, refURL, nil
+}
+
+// createImageTask creates a gemini_3_1_flash_image task and returns its id.
+func (c *Client) createImageTask(ctx context.Context, client tlsclient.HttpClient, token, teamID, prompt, aspectRatio, imageSize string, refImages []map[string]any) (string, error) {
+	opts := map[string]any{
+		"name":           "Nano Banana 2 - " + prompt,
+		"text_prompt":    prompt,
+		"aspect_ratio":   aspectRatio,
+		"num_images":     1,
+		"image_size":     imageSize,
+		"model":          "gemini-3.1-flash-image-preview",
+		"exploreMode":    false,
+		"creationSource": "tool-mode",
+	}
+	if len(refImages) > 0 {
+		opts["reference_images"] = refImages
+	}
+	res, err := c.apiJSON(ctx, client, token, teamID, http.MethodPost, "/v1/tasks", map[string]any{
+		"taskType":  "gemini_3_1_flash_image",
+		"options":   opts,
+		"asTeamId":  jsonNumberOrString(teamID),
+		"sessionId": uuid.NewString(),
+	})
+	if err != nil {
+		return "", err
+	}
+	task, _ := res["task"].(map[string]any)
+	id := strings.TrimSpace(stringValue(task["id"]))
+	if id == "" {
+		return "", fmt.Errorf("%w: image task missing id", ErrTemporaryUpstream)
+	}
+	return id, nil
+}