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

backend/app/main.py

@@ -1,4 +1,5 @@
 from fastapi import FastAPI, Depends
+from fastapi.staticfiles import StaticFiles
 from sqlalchemy.orm import Session
 from sqlalchemy import text
 from app.database import get_db
@@ -14,6 +15,9 @@ app = FastAPI(
     version="0.1.0",
 )
 
+# Serve generated recipe images from local filesystem.
+app.mount("/static", StaticFiles(directory="static"), name="static")
+
 
 @app.get("/health")
 def health_check(db: Session = Depends(get_db)):