test(realestate): add pytest harness with isolated SQLite fixture

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

realestate-lab/app/db.py

@@ -1,5 +1,6 @@
 # realestate-lab/app/db.py
 import json
+import os
 import sqlite3
 import logging
 from typing import Dict, Any, List, Optional
@@ -7,7 +8,7 @@ from datetime import date
 
 logger = logging.getLogger("realestate-lab")
 
-DB_PATH = "/app/data/realestate.db"
+DB_PATH = os.getenv("REALESTATE_DB_PATH", "/app/data/realestate.db")
 
 
 def _conn():