fix(related): skip empty id exclusion

[rissrice2105-agent]

Summary

• build the category fallback query before adding duplicate exclusions
• omit the PostgREST not.in filter when the tag query returns no IDs
• preserve duplicate filtering when one or two tag matches exist
• add a focused regression covering the zero-match category fallback

Validation

• focused route regression passed
• targeted TypeScript compilation passed
• git diff --check passed

Closes #7