feat(admin): documents, indexes and the plan seed

Adds admin's own documents, its unique indexes and the plan seed from
shared/license.

Licences are append-only -- a renewal writes a new row and supersedes the
old one -- because the history is the support tool. Plans are seeded with
$setOnInsert only, so a redeploy never stamps over staff edits to limits,
features or Paddle IDs.

admin_instances.instance_id unique is a correctness property, not an
optimisation: without it two customers could both claim one self-hosted
UUID and both be issued licences for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/go.mod

@@ -4,10 +4,12 @@ go 1.26.4
 
 require (
 	github.com/joho/godotenv v1.5.1
+	github.com/mrhid6/vantage/shared v0.0.0-00010101000000-000000000000
 	go.mongodb.org/mongo-driver/v2 v2.8.0
 )
 
 require (
+	github.com/hyperboloide/lk v0.0.0-20251220053519-b291812e3216 // 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