feat(import): bulk import supports nested subfolders

[Starosdev]

Feature Request

Upstream: Radarr#1170 (confirmed)

Environment

• Component: Backend / Import

---

Description

Bulk library import currently only scans the top level of the selected folder. It does not discover movies in nested subdirectories (e.g., organized by genre or year folders).

---

Acceptance Criteria

• Bulk import scans nested subdirectories
• Depth can be configured or is reasonable by default
• Performance is acceptable for deeply nested structures