Sticky routing: integration and e2e tests

Part of #393 (sticky routing). **Status: implemented** in #422 (commit `b5d3e15`), pending review/merge.

Seven Playwright API tests run the loop against the live stack (sequentially — the features API has a shared-row lost-update race under parallel enables, tracked separately):
- a recorded success pins the customer (decided gateway + `STICKY_ROUTING` label)
- `stickyRouting: false` per-request opt-out suppresses the pin
- no sticky behavior without the merchant feature flag
- a retried payment sticks to the connector that finally succeeded
- sticky state is scoped to the payment-method combo
- gateway failures erode the habit until the pin releases (net counts, floor 0)
- a late webhook with inline fields records without the decide-time snapshot

Unit coverage rides the earlier PRs (key encoding/case-folding, prune-victim selection, nested-map assembly, scoring-gate classifiers). Deeper Redis-level integration tests (health-veto trip, admission-budget rollover, TTL/prune mechanics, cardinality bench) moved to #417 so they aren't lost when this closes.