feat(standalone): support Postgres via DB_ENGINE env var

Lets a standalone install point at an existing Postgres database
(e.g. for testing against real data on a verification host) instead
of always writing to local SQLite. Defaults to SQLite when DB_ENGINE
is unset, so existing self-hosters see no behavior change.

pyproject.toml

@@ -40,6 +40,9 @@ standalone = [
     "python-dotenv",
     "whitenoise",
 ]
+postgres = [
+    "psycopg2-binary",
+]
 mcp = [
     "mcp-gate @ git+https://github.com/RunLit/mcp-gate.git@v0.1.0",
     "uvicorn",

standalone/settings.py

@@ -63,16 +63,29 @@ TEMPLATES = [
 
 WSGI_APPLICATION = "standalone.wsgi.application"
 
-# Database — SQLite for zero-setup deployment
+# Database — SQLite for zero-setup deployment, or Postgres when DB_ENGINE=postgresql
+# is set (e.g. to point a standalone install at an existing database).
 DATA_DIR = Path(os.environ.get("DATA_DIR", BASE_DIR / "data"))
 DATA_DIR.mkdir(parents=True, exist_ok=True)
 
-DATABASES = {
+if os.environ.get("DB_ENGINE") == "postgresql":
+    DATABASES = {
+        "default": {
+            "ENGINE": "django.db.backends.postgresql",
+            "NAME": os.environ["DB_NAME"],
+            "USER": os.environ["DB_USER"],
+            "PASSWORD": os.environ["DB_PASSWORD"],
+            "HOST": os.environ["DB_HOST"],
+            "PORT": os.environ.get("DB_PORT", "5432"),
+        }
+    }
+else:
+    DATABASES = {
         "default": {
             "ENGINE": "django.db.backends.sqlite3",
             "NAME": DATA_DIR / "db.sqlite3",
         }
-}
+    }
 
 # Password validation
 AUTH_PASSWORD_VALIDATORS = [