fix: keep separate processing/analysis queues, focus on batch AI tagging

Commit notes

Reverted unified processing queue - the separate queue architecture is intentional because: - Processing (preview/sequencer) is fast (~2-3s) → instant upload feel - Analysis (BPM, key, AI tagging) is slow (~10-30s) → background work

Retained optimizations: 1. Batch AI Tagging - accumulates requests, sends up to 10 samples per API call (80-90% reduction in AI API latency) 2. Loop Detection Early Exit - skips expensive analysis when metadata or filename clearly indicates loop status 3. Existing metadata skip for BPM/key detection

Updated PROCESSING_OPTIMIZATIONS.md with correct architecture explanation

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