[BUG] One batch evaluate call floods the Decision Audit with per-entry events

[prajjwalkumar17]

Summary

One evaluation call recorded one analytics event per batch entry, so the Decision Audit listed 30–52 "events" for a payment the router had called three times. Healthy batching read as a call storm, and the noise buried the routing mismatches the screen exists to surface. Traced on sandbox (pay_OR9TzfVd6AuKV8y7I9A8): three calls inside 31 ms batching 3, 7 and 7 entries → 3 request-hits + 17 per-entry events, before confirm or a checkout refresh added theirs.

Fix — in two steps

1. feat(analytics): count evaluation calls in the Decision Audit, not per-entry events #387 added call_count = uniq(request_id) beside event_count (Matches row reading 3 calls · 17 entries, per-call collapsible trace groups keyed on request_id) — at the cost of a ClickHouse column, migration 039, and column-drift probe machinery. It also fixed two things found on the way: an empty batch answered TE_04 400 — making every session/PML call from a card-only cut-over profile a logged failure plus a single-call retry — and now answers 200 {"results": []}; and POST /routing/evaluate/batch gained its missing classify_request arm so batch calls appear in the request-level audit at all.
2. revert(analytics): drop the call-count column and its migration machinery #390 + feat(analytics): one preview event per batch evaluate call #391 made the schema machinery unnecessary and removed it: a batch call now records one preview event per call — per-entry outcomes (pm type, status, gateway, rule name; stage + error for failures) ride in the event's details (~3 KB for a 12-entry batch vs the 64 KB truncation cap), payment_id is preserved when uniform across entries so audit lookup by payment id still lands, and any failed entry makes the call's status error. A 12-entry batch: 13 events before → 2 after. Event counts are proportional to calls again with no schema change and nothing to run on existing deployments — and the two ride-along fixes from feat(analytics): count evaluation calls in the Decision Audit, not per-entry events #387 are restored (the restored empty-batch arm even failed to compile against a plain revert, which is why feat(analytics): one preview event per batch evaluate call #391 targets the revert branch).

Verified A/B against main's binary with live ClickHouse; positional contract held in every batch response; routing + analytics Playwright suites 29/29 against the patched build.

PRs

• feat(analytics): count evaluation calls in the Decision Audit, not per-entry events #387
• revert(analytics): drop the call-count column and its migration machinery #390 — revert of the call-count machinery
• feat(analytics): one preview event per batch evaluate call #391 — one preview event per batch call

Part of #395.