Run Er Gate Endpoint

Run the entity-resolution acceptance gate INSIDE the live app process.

WHY in-app: entity_resolution_v3.confirm_pair's L3 arbiter needs a live LLM provider. Run standalone (docker exec … er_gate.py) it can't reach one, so every gray-band pair returns UNKNOWN/EXCEPTION and the health gate (correctly) goes red — the gate isn't fooled, but it also can't test real L3. Executed here, inside the process that already has the classifier provider configured, confirm_pair's L3 reaches the real model, so the gate exercises the true funnel: L1 deterministic synonyms (zero LLM) + L3 for the genuine gray band.

Deterministic L1 + tenant isolation already decide most canary pairs with no LLM, so this endpoint is cheap (only a handful of real L3 calls). READ-ONLY: it merely calls confirm_pair; it NEVER merges anything (merges stay behind the migration script's backup + interactive guards).

Body (optional): {"dataset": "<path>"} — defaults to the shipped canary set. Returns {status: PASS|FAIL, results: <gate json>, report: <human text>}.