feat: Define the API token scope vocabulary

Eight resources with read and write, write implying read. Coarse on
purpose: a scope per endpoint is a table nobody maintains, and a route
added without an entry either breaks or is unguarded.

server/internal/services/scopes.go

@@ -0,0 +1,92 @@
+package services
+
+import (
+	"errors"
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// ErrInvalidScope is returned when a token is requested with a scope outside
+// the vocabulary below.
+var ErrInvalidScope = errors.New("invalid scope")
+
+// ScopeResources is the whole vocabulary. Eight resources, each with :read and
+// :write, and write implies read on the same resource.
+//
+// It is deliberately coarse. A scope per endpoint is a table nobody maintains,
+// and a route added without an entry either fails closed and breaks, or
+// defaults open and is pointless.
+var ScopeResources = []string{
+	"servers",
+	"keys",
+	"secrets",
+	"workflows",
+	"monitors",
+	"vulns",
+	"workloads",
+	"settings",
+}
+
+const (
+	ScopeRead  = "read"
+	ScopeWrite = "write"
+)
+
+// AllScopes returns every valid scope string, sorted, for the API to advertise
+// to the token-creation UI.
+func AllScopes() []string {
+	out := make([]string, 0, len(ScopeResources)*2)
+	for _, r := range ScopeResources {
+		out = append(out, r+":"+ScopeRead, r+":"+ScopeWrite)
+	}
+	sort.Strings(out)
+	return out
+}
+
+func validScope(s string) bool {
+	resource, action, ok := strings.Cut(s, ":")
+	if !ok || (action != ScopeRead && action != ScopeWrite) {
+		return false
+	}
+	for _, r := range ScopeResources {
+		if r == resource {
+			return true
+		}
+	}
+	return false
+}
+
+// ValidScopes rejects an unknown scope and an empty list. A token with no
+// scopes can reach nothing, so creating one is a mistake worth naming rather
+// than a credential worth issuing.
+func ValidScopes(scopes []string) error {
+	if len(scopes) == 0 {
+		return fmt.Errorf("%w: at least one scope is required", ErrInvalidScope)
+	}
+	for _, s := range scopes {
+		if !validScope(s) {
+			return fmt.Errorf("%w: %q", ErrInvalidScope, s)
+		}
+	}
+	return nil
+}
+
+// ScopeSatisfied reports whether the held scopes cover the required one.
+// Holding "servers:write" satisfies a requirement of "servers:read"; the
+// converse is false.
+func ScopeSatisfied(held []string, required string) bool {
+	resource, action, ok := strings.Cut(required, ":")
+	if !ok {
+		return false
+	}
+	for _, h := range held {
+		if h == required {
+			return true
+		}
+		if action == ScopeRead && h == resource+":"+ScopeWrite {
+			return true
+		}
+	}
+	return false
+}