feat: add more reasonable graph credential config control flow (#496)

* use a default password (`"neo4j"`) matching the actual neo4j default
* separate default behavior for credentials and DB location. Previously
you'd have to set both to avoid default behavior. In practice it seems
like neo4j tends to think of them as separate though (credentials are
usually passed in tandem though so I didn't separate them further). So
now, you can pass your own DB location but use default
username/password, or vice versa.

Author: jsstevenson

server/src/metakb/database.py

@@ -54,21 +54,13 @@ def _get_credentials(
             uri = f"bolt://{secret['host']}:{secret['port']}"
             credentials = (secret["username"], secret["password"])
         else:
-            if all(
-                [
-                    "METAKB_DB_URL" in environ,
-                    "METAKB_DB_USERNAME" in environ,
-                    "METAKB_DB_PASSWORD" in environ,
-                ]
-            ):
-                uri = environ["METAKB_DB_URL"]
+            if not uri:
+                uri = environ.get("METAKB_DB_URL", "bolt://localhost:7687")
+            if (not credentials[0]) and (not credentials[1]):
                 credentials = (
-                    environ["METAKB_DB_USERNAME"],
-                    environ["METAKB_DB_PASSWORD"],
+                    environ.get("METAKB_DB_USERNAME", "neo4j"),
+                    environ.get("METAKB_DB_PASSWORD", "neo4j"),
                 )
-            else:  # local default settings
-                uri = "bolt://localhost:7687"
-                credentials = ("neo4j", "password")
     return uri, credentials