feat: Phase 6 - Bulk Operations implementation [unit: caching-strategies]

[jayfalls]

Summary

• Implements Phase 6 of the shared/caching package: Bulk Operations
• GetMany, SetMany, DeleteMany, DeletePattern, DeleteByTag all implemented
• 6 test cases covering all bulk operations

Files Changed

• backend/shared/caching/cache.go - Added 5 bulk operation methods
• backend/shared/caching/cache_test.go - Added 6 test cases

Phase 6 Complete

• ✅ GetMany returns map of hits only
• ✅ SetMany with maxSize validation and tag indexing
• ✅ DeleteMany removes multiple keys
• ✅ DeletePattern rejects bare *
• ✅ DeleteByTag removes entries by tag
• ✅ All testing.md test cases covered
• ✅ All tests pass

Next Steps

Phase 7: Invalidation Strategies