#34 FTX-34: config/db/errors/obs skeleton

app/config.py (all env in one place, one REDIS_URL, prod startup validation), app/db.py, app/errors.py + handlers with {error:{code,message,details,request_id}} shape (UB-API-6), middleware request_id (UB-API-5), JSON logging (UB-NF-7). Phase 0.