Add admin user management with is_active enforcement

Admin users page at /admin/users with paginated user list showing name,
email, team counts, role, join date, and active status toggle. Inactive
users are blocked from all authenticated endpoints immediately via DB
check in JWT middleware. OAuth login errors now show human-readable
messages on the login page.

internal/db/models.go

@@ -197,6 +197,8 @@ type User struct {
 	IsAdmin      bool               `json:"is_admin"`
 	CreatedAt    pgtype.Timestamptz `json:"created_at"`
 	UpdatedAt    pgtype.Timestamptz `json:"updated_at"`
+	IsActive     bool               `json:"is_active"`
+	DeletedAt    pgtype.Timestamptz `json:"deleted_at"`
 }
 
 type UsersTeam struct {