Add tags CRUD and testimonial tagging

[arferreira]

Summary

• Tags CRUD: list, create, update, delete — scoped to projects with ownership enforcement
• Testimonial tagging via PUT /api/v1/testimonials/:id/tags (replace-all semantics)
• Testimonial responses now include a tags array (batch-loaded for list endpoints)
• 12 integration tests covering CRUD, uniqueness constraints, cross-project rejection, and auth

Test plan

• cargo check passes
• cargo clippy -- -D warnings clean
• cargo fmt applied
• All 38 tests pass (12 new tags + 10 testimonials + 8 projects + 8 auth)
• rapina openapi export + rapina openapi check pass