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",