fix: Use os.environ[] instead of .get() for DATABASE_URL

Fixes Pylance type error - create_engine() expects str, not str | None.
Using direct access raises KeyError if not set, which is correct behavior.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

notebooks/housing/rent_trend_line.ipynb

@@ -38,7 +38,7 @@
     "# Load .env from project root\n",
     "load_dotenv('../../.env')\n",
     "\n",
-    "engine = create_engine(os.environ.get('DATABASE_URL'))\n",
+    "engine = create_engine(os.environ['DATABASE_URL'])\n",
     "\n",
     "# City-wide average rent by year\n",
     "query = \"\"\"\n",