Integrate Radashi utility library across codebase

Commit notes

Replaced custom utility patterns with Radashi functions:

sleep(): - backend/src/queue/index.ts: 5 occurrences - frontend/src/routes/library/+page.svelte: 1 occurrence - frontend/src/lib/stores/audio.svelte.ts: 1 occurrence

clamp(): - frontend/src/lib/components/BpmControl.svelte: 2 occurrences - frontend/src/lib/stores/sequencer.svelte.ts: 1 occurrence - backend/src/utils/sanitize.ts: 1 occurrence

unique(): - backend/src/services/sample-cleanup.ts: 2 occurrences - backend/src/services/user-cleanup.ts: 2 occurrences - backend/src/services/orphan-cleanup.ts: 1 occurrence - backend/src/services/ai-tagging.ts: 2 occurrences - backend/src/worker.ts: 1 occurrence

sum(): - frontend/src/lib/api/upload.ts: 1 occurrence - frontend/src/lib/stores/subscription.svelte.ts: 1 occurrence - frontend/src/lib/components/PackEditDrawer.svelte: 2 occurrences - frontend/src/routes/library/components/UploadDropzone.svelte: 2 occurrences

capitalize(): - backend/src/services/ai-copywriting.ts: removed custom function, using radashi

Total impact: ~50+ lines of cleaner, more maintainable code

Co-authored-by: armin.naimi <[email redacted]>