Update environment variables and enhance Docker configurations for improved deployment

- Modified .env.portainer to include new environment variables for S3 private bucket URL, Nostr JWT secrets, and SendGrid options.
- Updated docker-compose.yml to support the new environment variables, enhancing service configurations.
- Added a seed content script to the backend package.json for initializing the database with sample data.
- Refactored helper functions to construct S3 URLs more robustly, accommodating potential missing configurations.
- Enhanced dev.sh script to seed the database if empty, ensuring content availability during development.

scripts/dev.sh

@@ -284,6 +284,14 @@ for i in $(seq 1 60); do
   sleep 2
 done
 
+# Seed database if empty (so catalog and films appear; user uploads require re-upload after DB reset)
+PROJECT_COUNT=$(curl -s "http://localhost:$BACKEND_PORT/projects" 2>/dev/null | grep -o '"id"' | wc -l | tr -d ' ')
+if [ "$PROJECT_COUNT" = "0" ] || [ -z "$PROJECT_COUNT" ]; then
+  echo -e "${CYAN}  Database has no projects — seeding content...${NC}"
+  (cd "$BACKEND_DIR" && set -a && [ -f .env ] && . ./.env && set +a && npm run seed:content 2>&1 | prefix_output "seed-db" "$CYAN") || true
+  echo -e "${GREEN}  Seed done. Reload the app to see the catalog.${NC}"
+fi
+
 # ═════════════════════════════════════════════════════════════
 # STEP 4 — Frontend dev server
 # ═════════════════════════════════════════════════════════════