Add caching layer for vocabulary fetching during API startup

[SarthakJagota]

Summary

This PR introduces a lightweight caching mechanism for vocabulary fetching during API startup.

Currently, vocabularies are fetched from GitHub on every startup, which can slow initialization and create a dependency on network availability.

Changes

• Added cache.py helper to store and load vocabularies locally
• Load vocabularies from cache when available
• Fetch and persist vocabularies when cache is missing
• Added fallback to cached vocabularies if fetching fails
• Added logging to clarify when cache vs remote fetch is used

Impact

• Faster startup for local development
• Improved reliability when external services are unavailable
• No change to existing API behavior

Notes

Cached vocabularies can be refreshed by removing the .cache directory.

Summary by Sourcery

Introduce a local caching mechanism for vocabulary data during API startup to reduce reliance on remote fetching and improve startup performance.

New Features:

• Add a cache helper module to persist and load vocabularies from a local .cache directory.
• Load vocabularies from the local cache on startup when available instead of always fetching from GitHub.

Enhancements:

• Add a fallback to use cached vocabularies when remote fetching fails to improve startup robustness.
• Add logging to indicate whether vocabularies are loaded from cache or fetched remotely.

#534

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