[DwC export]: Add mechanism to clean/remove orphaned cache tables

[grantfitzsimmons]

Goal

Ensure that when a schema mapping or extension is deleted, its associated cache table(s) are removed from the database automatically.

Background

Orphaned tables waste storage and cause confusion. Cleanup must be reliable even if deletion happens outside normal UI flows.

Acceptance Criteria

• Deleting a schemamapping record triggers removal of its cache table (e.g., DROP TABLE IF EXISTS dwc_fish).
• Deleting an extension mapping removes its cache table (e.g., dwc_fish_ext1_audiovisual).
• A periodic cleanup mechanism (or on-startup check) identifies and removes cache tables with no corresponding schemamapping record.