Update app.py

perf: remove concurrency_limit on create API

perf: in-memory feed cache — load once at startup, append on upload, zero disk reads

Revert "perf: single index.json for community feed (1 read vs N meta.json reads)"

perf: single index.json for community feed (1 read vs N meta.json reads)

fix: check actual thumb.png file instead of has_thumb metadata flag

ace-step-jam: AI music generation studio

feat: ACE-Step Studio — custom frontend for ACE-Step v1.5 music generation