v1.9.36 — Live audio preview

New POST /preview/audio route renders a short slice of a clip with an FFmpeg filter applied and returns WAV bytes — denoise, normalize, compressor, EQ, and silence-preview all wired on the backend. First UI hook is a 'Preview 10s' button under Audio → Denoise's strength slider. Replaces the guess-wait-compare-retry loop with one-click A/B. Additional UI buttons for the other 4 filters are follow-up work.