Redesign document approval flow

Replace the per-approver add/remove model with a quorum-based approval
system. Documents now have default approvers that are pre-populated when
requesting approval, and the publish dialog lets users adjust the list
before submitting.

Key changes:

- Add PENDING_APPROVAL document version status with dedicated transitions
- Introduce approval quorums with request/approve/reject/void lifecycle
- Add default approvers per document (stored in document_default_approvers)
  with MERGE-based upsert for efficient sync
- Add NoDuplicates validator for slice fields
- Split ALTER TYPE ADD VALUE migrations into separate files (required by
  PostgreSQL when run inside transactions)
- Use VOIDED consistently for both quorum status and decision state enums
- Expose void/approve/reject through GraphQL and MCP, with e2e tests
- Add approval management UI: publish dialog with approver selection,
  approval list with void support, and external approve/reject page

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/validator/validator_common.go

@@ -47,6 +47,36 @@ func Required() ValidatorFunc {
 	}
 }
 
+// NoDuplicates validates that a slice contains no duplicate elements.
+func NoDuplicates() ValidatorFunc {
+	return func(value any) *ValidationError {
+		actualValue, isNil := dereferenceValue(value)
+		if isNil {
+			return nil
+		}
+
+		rv := reflect.ValueOf(actualValue)
+		if rv.Kind() != reflect.Slice {
+			return newValidationError(ErrorCodeInvalidFormat, "value must be a slice")
+		}
+
+		if !rv.Type().Elem().Comparable() {
+			return newValidationError(ErrorCodeInvalidFormat, "slice elements must be comparable")
+		}
+
+		seen := make(map[any]struct{}, rv.Len())
+		for i := range rv.Len() {
+			elem := rv.Index(i).Interface()
+			if _, ok := seen[elem]; ok {
+				return newValidationError(ErrorCodeInvalidFormat, "must not contain duplicates")
+			}
+			seen[elem] = struct{}{}
+		}
+
+		return nil
+	}
+}
+
 // NotEmpty validates that a field is not empty.
 // Similar to Required, but can be used independently.
 func NotEmpty() ValidatorFunc {