feat: config-driven environments (env vars + start scripts)

Add env var overrides matching main app pattern:
- CANTEEN_IMPORT_PORT — port override (default 8091)
- CANTEEN_IMPORT_DB_PATH — database path override
- CANTEEN_IMPORT_SEED_DIR — seed data directory override
- CANTEEN_IMPORT_UPLOAD_DIR — upload directory override

Create start scripts in repo:
- start.sh — production (default port 8091)
- start-dev.sh — development (port 8093, assets.dev.db)

Fix: dev clone now pulls from Gitea (not local prod dir)

start.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+# 🏭 Start Seed Import — Production
+set -euo pipefail
+
+cd "$(dirname "$0")"
+PROJECT_DIR="$(pwd)"
+
+echo "🚀 Starting Seed Import (prod) on port ${CANTEEN_IMPORT_PORT:-8091}..."
+echo "   DB:  ${CANTEEN_IMPORT_DB_PATH:-$PROJECT_DIR/seed-data/assets.db}"
+echo ""
+
+exec python3 web/server.py