feat: stream gpt-image generation via SSE with keepalive

- /api/generate now responds with text/event-stream end-to-end
- forwards upstream image_generation.* / image_edit.* partial+completed events
- 20s keepalive comments survive Cloudflare's 120s proxy-read timeout
- falls back to non-streaming when upstream rejects stream/partial_images
- drops @ai-sdk/openai-compatible, @ai-sdk/react, ai (unused)
- frontend consumes SSE via fetch+ReadableStream, shows progressive preview

package.json

@@ -14,9 +14,6 @@
 		"typescript": "^6.0.3"
 	},
 	"dependencies": {
-		"@ai-sdk/openai-compatible": "^2.0.47",
-		"@ai-sdk/react": "^3.0.186",
-		"ai": "^6.0.184",
 		"react": "^19.2.6",
 		"react-dom": "^19.2.6"
 	}