feat: implement core pixel canvas with real-time updates

Next.js App Router + Upstash Redis BITFIELD for 2048x2048 canvas.
- 32-color palette (5-bit encoding) from rplace.live
- Batch pixel placement API (up to 256 pixels per request)
- Stackable credit rate limiting via Redis Lua script
- SSE real-time broadcasting via Redis sorted set
- HTML5 Canvas frontend with zoom/pan and color picker

.env.example

@@ -0,0 +1,8 @@
+UPSTASH_REDIS_REST_URL=
+UPSTASH_REDIS_REST_TOKEN=
+NEXTAUTH_URL=http://localhost:3000
+NEXTAUTH_SECRET=
+GOOGLE_CLIENT_ID=
+GOOGLE_CLIENT_SECRET=
+GITHUB_CLIENT_ID=
+GITHUB_CLIENT_SECRET=