feat: add health checks, OG image generation, and R2 integration

- Implement health check system with caching and singleflight pattern
- Add OG image generation via Satori with R2 storage backend
- Configure Railway deployment with health check endpoint
- Add connection pooling and Unix socket support for Bun SSR
- Block external access to internal routes (/internal/*)

railway.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://railway.com/railway.schema.json",
+  "deploy": {
+    "healthcheckPath": "/api/health",
+    "healthcheckTimeout": 30
+  }
+}