fix(ci): switch from GHA cache to registry cache

[MaxAnderson95]

Summary

Switch from GitHub Actions cache to registry cache to fix build caching.

Root Cause

The previous caching attempts failed because:

• 10 GB GHA cache limit exceeded with 14 parallel builds (7 services × 2 platforms)
• Cache eviction thrashing: new cache entries evicted old ones before they could be used
• The power-service-amd64 cache index was being evicted almost immediately after creation

Solution

Use registry cache instead of GHA cache:

• Stores build cache as tags in ghcr.io (e.g., image:cache-amd64)
• No practical size limit (uses container registry storage)
• Each service gets its own cache tag that persists indefinitely

Changes

# Before (GHA cache - 10GB limit shared across all builds)
cache-from: type=gha,scope=power-service-amd64
cache-to: type=gha,mode=max,scope=power-service-amd64

# After (Registry cache - unlimited, stored in ghcr.io)
cache-from: type=registry,ref=ghcr.io/.../power-service:cache-amd64
cache-to: type=registry,ref=ghcr.io/.../power-service:cache-amd64,mode=max