feat(multi-keys-gsi): consolidate all SGIs into one

[hackmajoris]

Optimize DynamoDB Design: Reduce GSIs from 3 to 1

Summary

Restructured DynamoDB table design to use EntityType as partition key and consolidated skill GSIs, reducing infrastructure costs by eliminating 2 GSIs while maintaining optimal query performance.

Changes Made

Infrastructure (CDK)

• Main Table: Changed from entity_id (PK) to EntityType (PK) + entity_id (SK)
• Removed GSIs: Eliminated ByEntityType and BySkillLevel GSIs
• Consolidated GSI: BySkill now handles both skill-only and skill+level queries using composite sort keys

Database Layer

• Updated all GetItem/DeleteItem operations to use both EntityType and entity_id keys
• Modified Query operations to use main table for entity type queries (no GSI needed)
• Consolidated skill queries to use single BySkill GSI with multi-key sort structure

Model Updates

• Added composite sort key field SkillCompositeSort to UserSkill model
• Updated SetKeys() methods to build composite keys for GSI queries