Restructure src/ into lambda/handlers/repositories/shared layers

[Valamorde]

Summary

Reorganises the entire src/ tree into clear architectural layers so the directory structure reflects the responsibilities of the code inside it.

Before:

src/
  connect/, disconnect/, keepalive/
  sendmessage/handlers/
  services/actions/
  shared/repositories/

After:

src/
  lambda/          # Entry points — one file per Lambda function
  handlers/
    messages/      # WebSocket message handlers — one per eventType
    services/      # HTTP action handlers — one per action
  repositories/    # All DynamoDB access (connection, scene, campaign)
  shared/          # Cross-cutting: types, constants, db/apigw singletons, key builders

• git mv used throughout to preserve rename history
• CDK stack entry paths updated to reflect new locations
• ServiceContext centralised in src/shared/types.ts
• Vitest @/ alias updated; all tests relocated to mirror the new structure

Test plan

• pnpm test — all tests pass with no path resolution errors
• cdk synth — no infrastructure changes