feat(admin): module skeleton, config and two database connections

Adds the admin module: env config with fail-fast validation, two MongoDB
connections (its own vantage_admin database plus a narrow path into the
control plane), the boot sequence and the image.

Config refuses to start without a signing key, and both Mongo URIs must
name their database inline -- admin talks to two databases, so a bare
MONGO_DB would be ambiguous about which.

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

admin/go.mod

@@ -0,0 +1,21 @@
+module github.com/mrhid6/vantage/admin
+
+go 1.26.4
+
+require (
+	github.com/joho/godotenv v1.5.1
+	go.mongodb.org/mongo-driver/v2 v2.8.0
+)
+
+require (
+	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
+	github.com/xdg-go/stringprep v1.0.4 // indirect
+	github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78 // indirect
+	golang.org/x/crypto v0.54.0 // indirect
+	golang.org/x/sync v0.22.0 // indirect
+	golang.org/x/text v0.40.0 // indirect
+)
+
+replace github.com/mrhid6/vantage/shared => ../shared