feat: implement cmd management for targets, LUNs, and TPGTs

[carmark]

Summary

Closes #36

• Fix target delete URL path bug: client sent DELETE to /targets/<name> but server route was /target/{name}
• Implement target create/delete server handlers: previously no-op stubs, now fully functional with validation and proper HTTP status codes
• Add DeleteTarget to SCSI service: with force flag support and mutex locking for thread safety
• Implement full LU management: create/list/delete through all layers (CLI -> API client -> server -> SCSI service)
• Add TPGT list command: gotgt list tpgt --target <name> to show target portal group tags
• Add unit tests: 14 new tests covering target router handlers, LU router handlers, and SCSI service DeleteTarget

New CLI commands

gotgt create lu --target <name> --lun <n> --device-id <id> --path <path> [--block-shift <n>]
gotgt list lu --target <name>
gotgt list tpgt --target <name>
gotgt rm lu --target <name> --lun <n>

Test plan

• go build ./... passes
• go vet ./... passes
• gofmt -l . clean
• go test -v ./pkg/... ./mock/... - all 47 tests pass
• CI pipeline (build + unit tests + libiscsi functional tests)

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