feat(backend): implement database schema, models, and import workflow

- Add ProxyHost, CaddyConfig, RemoteServer, SSL, AccessList, User, Setting, ImportSession models
- Implement ProxyHostService and RemoteServerService with domain/name uniqueness validation
- Add Caddyfile import handler with conflict detection and user review workflow
- Create ProxyHostHandler and RemoteServerHandler with full CRUD operations
- Wire up Gin/GORM/SQLite dependencies and create missing internal packages
- Add database connection layer, server routing, and version info
- Update routes to register all new handlers and auto-migrate models
- Configure import environment variables and mount points

Addresses Issue #5 (data persistence) and Issue #43 (remote servers)
Backend now compiles cleanly with go build

backend/internal/server/server.go

@@ -0,0 +1,21 @@
+package server
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// NewRouter creates a new Gin router with frontend static file serving.
+func NewRouter(frontendDir string) *gin.Engine {
+	router := gin.Default()
+
+	// Serve frontend static files
+	if frontendDir != "" {
+		router.Static("/assets", frontendDir+"/assets")
+		router.StaticFile("/", frontendDir+"/index.html")
+		router.NoRoute(func(c *gin.Context) {
+			c.File(frontendDir + "/index.html")
+		})
+	}
+
+	return router
+}