[FEAT] Add an importer framework (CSV/Jira/GitHub bulk import)

[martian56]

Type / Priority

Feature / Medium

Summary

There is no way to import existing work items (CSV, Jira, or bulk GitHub) into a project.

Current state (evidence)

• No importer model, service, handler, routes, or UI anywhere in api/ or ui/src/. GitHub sync is push/webhook-based only (no bulk pull import); no import status/progress tracking.

Proposed scope

Add a generic importer model (service, status, progress, config) with async import via RabbitMQ; start with a CSV parser, then Jira/GitHub bulk import; add an import-progress UI.