Clears the Redis policy cache and invalidates local caches in ALL Core-API instances.

Propagation Mechanism:

1. Deletes policy keys from Redis (L2 shared cache)
2. Publishes Redis Pub/Sub event to "policy-cache-updates" channel
3. All Core-API instances receive event and invalidate their L1 caches (Caffeine)
4. Next cryptographic operation will reload fresh policies from Redis or database

Behavior:
• Without tenantId parameter: Clears entire policy cache (all tenants + deployment)
• With tenantId parameter: Clears only that specific tenant's policies

Use Cases:
• Emergency cache invalidation across entire cluster
• Policies updated directly in database (bypassing Admin API)
• Cache corruption suspected
• Testing policy changes in production
• Troubleshooting policy inconsistencies
• Forcing immediate policy refresh cluster-wide

Performance Impact:
• Propagation latency: ~10-15ms via Redis Pub/Sub
• Next operations will experience cache miss (slower initial request)
• Automatic cache repopulation on demand

Note: This operation only clears cache. To reload policies from database,
use the POST /sync endpoint after clearing cache.