Split schema.graphql and schema.resolvers.go into per-domain files

Move Query/Mutation field declarations from the monolithic schema.graphql
into each domain's .graphql file using extend type, so gqlgen places
resolvers in the matching *.resolvers.go files. Extract helper functions
into *_helpers.go files to prevent gqlgen from commenting them out.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/graph/activity.resolvers.go

@@ -7,7 +7,9 @@ package graph
 
 import (
 	"context"
+	"fmt"
 
+	"adam-french.co.uk/backend/graph/model"
 	"adam-french.co.uk/backend/models"
 )
 
@@ -16,6 +18,33 @@ func (r *activityResolver) ID(ctx context.Context, obj *models.Activity) (int, e
 	return int(obj.ID), nil
 }
 
+// CreateActivity is the resolver for the createActivity field.
+func (r *mutationResolver) CreateActivity(ctx context.Context, input model.CreateActivityInput) (*models.Activity, error) {
+	if !IsAdminFromCtx(ctx) {
+		return nil, fmt.Errorf("admin access required")
+	}
+
+	activity := models.Activity{Type: input.Type, Name: input.Name, Link: input.Link}
+	if err := r.Store.DB.Create(&activity).Error; err != nil {
+		return nil, err
+	}
+
+	return &activity, nil
+}
+
+// Activities is the resolver for the activities field.
+func (r *queryResolver) Activities(ctx context.Context) ([]*models.Activity, error) {
+	var activities []models.Activity
+	if err := r.Store.DB.Order("created_at DESC").Find(&activities).Error; err != nil {
+		return nil, err
+	}
+	result := make([]*models.Activity, len(activities))
+	for i := range activities {
+		result[i] = &activities[i]
+	}
+	return result, nil
+}
+
 // Activity returns ActivityResolver implementation.
 func (r *Resolver) Activity() ActivityResolver { return &activityResolver{r} }