feat(sharedb): add document lifecycle events

[edocevol]

Summary

• add EventType values for operation and delete lifecycle events
• add Backend.DeleteDoc with memory and Redis implementations
• add Server.DeleteDocument with exact version validation and delete event publication
• ensure Redis delete removes snapshot and op log together to avoid stale history on recreate
• document DeleteDocument behavior in sharedb README

Test Plan

• go test ./...
• cd sharedb && go test ./...
• cd sharedb && go vet ./...
• cd sharedb && go test -race ./...

Review

• independent reviewer subagent: PASS