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/service/admin_read.go

@@ -342,6 +342,11 @@ func (s *AdminReadService) Providers(ctx context.Context) ([]map[string]any, err
 	}{
 		{Name: "chatgpt", Pool: "chatgpt", Type: "openai"},
 		{Name: "adobe", Pool: "adobe", Type: "adobe"},
+		{Name: "runway", Pool: "runway", Type: "runway"},
+		{Name: "leonardo", Pool: "leonardo", Type: "leonardo"},
+		{Name: "krea", Pool: "krea", Type: "krea"},
+		{Name: "imagine", Pool: "imagine", Type: "imagine"},
+		{Name: "grok", Pool: "grok", Type: "grok"},
 	}
 	out := make([]map[string]any, 0, len(providers))
 	for _, item := range providers {