feat(aws): complete the RDS adapter (create/delete + snapshots)

[hectorvent]

Complete the AWS RDS adapter

RDS is already registered under the database type but is read-only (list + inspect). This issue
brings it to full CRUD.

• Add create / delete instance to packages/api/src/adapter-aws/AwsRdsAdapter.ts (+ test)
• Surface snapshot list / create (legacy routes/rds.ts already exposes these — reuse or port)
• Update the schema actions / capabilities.resourceActions so the create form + delete button appear
• Verify against the local emulator (floci/src/main/java/io/github/hectorvent/floci/services/rds/)