feat: add FastAPI lifespan and healthcheck endpoint

[paul-paliychuk]

[!IMPORTANT] Add FastAPI lifespan for graphiti initialization, a /healthcheck endpoint, and initialize_graphiti function for index setup.

• FastAPI Lifespan: Added lifespan async context manager in main.py for graphiti initialization.
• Endpoints: Replaced root endpoint with /healthcheck in main.py, returning JSON status.
• Graphiti Initialization: Added initialize_graphiti() in zep_graphiti.py for index and constraint setup.

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