Use embedded postgres db to simplify self-hosted setup (#1141)

Use pgserver python package as an embedded postgres db,
installed directly as a khoj python package dependency.

This significantly simplifies self-hosting with just a `pip install khoj'. 
No need to also install postgres separately.

Still use standard postgres server for multi-user, production use-cases.

pyproject.toml

@@ -70,6 +70,7 @@ dependencies = [
     "itsdangerous == 2.1.2",
     "httpx == 0.28.1",
     "pgvector == 0.2.4",
+    "pgserver == 0.1.4",
     "psycopg2-binary == 2.9.9",
     "lxml == 4.9.3",
     "tzdata == 2023.3",