feat(admin): cloud owner login against the control plane

Cloud customers sign in with the control-plane credentials they already
hold, so there is no second password to manage. Two accepted consequences,
documented at the handler: their control-plane password now also unlocks
billing, and only role owner may sign in -- admin and member are refused
because billing is an owner concern.

A self-hosted customer_users row wins over a control-plane user with the
same address, so the precedence is chosen rather than emergent.

Adds the reads of control users this needs; the write surface is still one
UpdateOne on instances.

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

admin/internal/auth/cloud.go

@@ -0,0 +1,94 @@
+package auth
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+	"github.com/mrhid6/vantage/admin/internal/audit"
+	"github.com/mrhid6/vantage/admin/internal/db"
+	adminmodels "github.com/mrhid6/vantage/admin/internal/models"
+	sharedmodels "github.com/mrhid6/vantage/shared/models"
+	"go.mongodb.org/mongo-driver/v2/bson"
+	"golang.org/x/crypto/bcrypt"
+)
+
+// HandleCloudLogin authenticates a cloud customer against the CONTROL PLANE's
+// users collection, with the credentials they already have.
+//
+// Two consequences worth stating plainly, because they are real and were
+// accepted deliberately:
+//
+//  1. A cloud user's control-plane password now also unlocks billing. Any
+//     password change or compromise has a wider blast radius than before.
+//  2. Only control-plane role "owner" may sign in here. admin and member are
+//     refused — billing is an owner concern.
+//
+// Mitigations: rate limits, an identical error for every failure, and an audit
+// entry for every attempt.
+func HandleCloudLogin(c *gin.Context) {
+	var body struct {
+		Email    string `json:"email"`
+		Password string `json:"password"`
+	}
+	if err := c.ShouldBindJSON(&body); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "email and password are required"})
+		return
+	}
+	email := strings.ToLower(strings.TrimSpace(body.Email))
+	ctx := c.Request.Context()
+
+	if !allowAttempt(email, c.ClientIP()) {
+		c.JSON(http.StatusTooManyRequests, gin.H{"error": "too many attempts, try again later"})
+		return
+	}
+
+	reject := func(reason string) {
+		audit.Write(ctx, adminmodels.AuditEntry{
+			Actor: email, Action: "cloud.login_failed", IP: c.ClientIP(), Detail: reason})
+		c.JSON(http.StatusUnauthorized, gin.H{"error": genericAuthError})
+	}
+
+	// A self-hosted customer_users row wins over a control-plane user with the
+	// same address. Documented so the behaviour is chosen rather than emergent.
+	if n, _ := db.Admin("customer_users").CountDocuments(ctx, bson.M{"email": email}); n > 0 {
+		HandleCustomerLogin(c)
+		return
+	}
+
+	var u sharedmodels.User
+	if err := db.Control("users").FindOne(ctx, bson.M{"email": email}).Decode(&u); err != nil {
+		bcrypt.CompareHashAndPassword([]byte(dummyHash), []byte(body.Password))
+		reject("unknown email")
+		return
+	}
+	if bcrypt.CompareHashAndPassword([]byte(u.PasswordHash), []byte(body.Password)) != nil {
+		reject("bad password")
+		return
+	}
+	if u.Role != sharedmodels.RoleOwner {
+		reject("role " + u.Role + " is not permitted")
+		return
+	}
+
+	// Resolve the admin-side account that owns this user's instance.
+	var inst adminmodels.Instance
+	if err := db.Admin("admin_instances").FindOne(ctx,
+		bson.M{"instance_id": u.InstanceID}).Decode(&inst); err != nil {
+		reject("no account for instance " + u.InstanceID)
+		return
+	}
+
+	id, err := Save(ctx, Session{
+		UserID: u.UserID, Kind: KindCustomer, Email: u.Email, AccountID: inst.AccountID,
+	})
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"error": "session failed"})
+		return
+	}
+	SetCookie(c, id)
+	clearAttempts(email)
+	audit.Write(ctx, adminmodels.AuditEntry{
+		Actor: email, Action: "cloud.login", AccountID: inst.AccountID, IP: c.ClientIP()})
+	c.JSON(http.StatusOK, gin.H{"kind": KindCustomer, "email": u.Email})
+}

admin/internal/auth/customer.go

@@ -0,0 +1,12 @@
+package auth
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// HandleCustomerLogin is replaced by the real self-hosted login in Task 7.
+func HandleCustomerLogin(c *gin.Context) {
+	c.JSON(http.StatusNotImplemented, gin.H{"error": "not implemented yet"})
+}