fix(neo4j): create missing neo4j_connection module with basic_auth

Health check imports app.core.neo4j_connection._driver but the module
did not exist, causing neo4j health to always return False.

app/core/neo4j_connection.py

@@ -0,0 +1,28 @@
+"""Neo4j driver singleton for health checks and graph queries."""
+from __future__ import annotations
+
+import logging
+import os
+
+log = logging.getLogger(__name__)
+_driver = None
+
+
+def _init() -> None:
+    global _driver
+    if _driver is not None:
+        return
+    try:
+        from neo4j import GraphDatabase, basic_auth
+
+        uri = os.getenv("NEO4J_URI", "bolt://rmi-neo4j:7687")
+        user = os.getenv("NEO4J_USER", "neo4j")
+        password = os.getenv("NEO4J_PASSWORD", "password")
+        _driver = GraphDatabase.driver(uri, auth=basic_auth(user, password))
+        log.info("neo4j_driver_connected")
+    except Exception as exc:
+        log.warning("neo4j_driver_init_failed err=%s", exc)
+        _driver = None
+
+
+_init()