feat: Abverkauf velocity engine — sql/118 + analyzer + API endpoints
- sql/118-stock-velocity.sql: new stock_velocity (UPSERT per tx×vendor)
and stock_velocity_events tables with TimescaleDB-compatible indexes
- stock-velocity-analyzer.ts: computes sell-through from stock_observations
time-series; detects sold/zulauf/data_gap events, trims top-10% outliers,
predicts stockout date, assigns high/medium/low/insufficient confidence
- scheduler.ts: analyze:stock:velocity job at 04:30/12:30/20:30 UTC
- stock.ts: GET /api/stock/velocity (paginated, filterable by vendor/confidence/
stockout_days) + GET /api/stock/velocity/:id (per-product with event history)
- First run: 208 products, 979 sell events, 2811 Zulauf events written
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