feat(backend): wire exclude_recipe_ids, verify MealPlan votes schema, add image generation service

- api/meal_plans.py: /regenerate now passes exclude_recipe_ids into generate_meal_plan
- planner/generate.py: filter recipe_dicts by exclude_recipe_ids set
- image_generation.py: OpenAI gpt-image-1 client with prompt building, b64_json handling
- main.py: StaticFiles mount at /static for generated images
- admin.py: POST /api/admin/trigger-images endpoint for batch generation
- scripts/generate_images.py: CLI for batch image generation
- docker-compose.yml + nginx: volume mounts for static/images persistence
- Verify MealPlanItem.votes ↔ MealPlanVote relationship is correct; no model bug exists

nginx/nginx.conf

@@ -21,6 +21,13 @@ http {
             proxy_cache_bypass $http_upgrade;
         }
 
+        location /static/ {
+            set $backend_upstream http://backend:8000;
+            proxy_pass $backend_upstream;
+            proxy_http_version 1.1;
+            proxy_set_header Host $host;
+        }
+
         location / {
             set $frontend_upstream http://frontend:80;
             proxy_pass $frontend_upstream;