feat: user events handler partial recovery

[aintnostressin]

Prepares user handlers for partial recovery.

• Reorders the user DEL handler to delete the graph node last (after Redis/search cleanup), so that a retry after partial failure re-enters the CreatedOrDeleted branch and re-runs idempotent Redis DEL/ZREM ops before finally removing the graph node.
• Replaces UserDetails::delete() with remove_from_index_multiple_json() which is a pure Redis DEL — idempotent and safe to re-run without needing the graph node.
• Adds two integration tests: full idempotent retry (expects SkipIndexing) and graph-last partial-failure recovery.