[Performance] Optimize cache key generation using hashed scopes

[cbcoutinho]

Context

From PR #401 code review - Medium priority performance optimization.

Current Implementation

Cache keys are generated by joining sorted scopes with comma:

cache_key = f"{user_id}:background:{','.join(sorted(required_scopes))}"

Problem

For requests with many scopes, this creates long cache keys. While functional, hashing would be more efficient.

Proposed Solution

Use hash of sorted scopes instead:

import hashlib
scope_hash = hashlib.sha256(','.join(sorted(required_scopes)).encode()).hexdigest()[:16]
cache_key = f"{user_id}:background:{scope_hash}"

Benefits

• Shorter cache keys (fixed 16 chars vs variable length)
• Faster cache lookups with many scopes
• Reduced memory footprint in cache storage

References

• File: nextcloud_mcp_server/auth/token_broker.py:323
• Related to PR feat(astrolabe): Nextcloud app UI with PDF viewer, webhooks, and OAuth refresh #401 security fixes

Priority

Medium - Performance optimization, no correctness impact