refactor(sitesvc): use shared models and provision

Deletes internal/provision and the hand-mirrored Org and User structs. The
control plane and sitesvc now share one definition of both.

sitesvc/go.mod

@@ -1,16 +1,15 @@
 module github.com/mrhid6/vantage/sitesvc
 
-go 1.26
+go 1.26.4
 
 require (
 	github.com/google/uuid v1.6.0
 	github.com/joho/godotenv v1.5.1
-	go.mongodb.org/mongo-driver/v2 v2.2.2
+	go.mongodb.org/mongo-driver/v2 v2.8.0
 	golang.org/x/crypto v0.54.0
 )
 
 require (
-	github.com/golang/snappy v1.0.0 // indirect
 	github.com/klauspost/compress v1.17.6 // indirect
 	github.com/xdg-go/pbkdf2 v1.0.0 // indirect
 	github.com/xdg-go/scram v1.2.0 // indirect
@@ -19,3 +18,5 @@ require (
 	golang.org/x/sync v0.22.0 // indirect
 	golang.org/x/text v0.40.0 // indirect
 )
+
+replace github.com/mrhid6/vantage/shared => ../shared