Add per-record diff details to the reconciliation endpoint

[therealjhay]

Current behavior:
Reconcile endpoint returns count comparison only — no per-record details.

Expected behavior:
Add detail query parameter. When true, compare all settlement IDs pairwise and return mismatches: [{ id, field, gatewayValue, engineValue }]. Cap at 100. Add truncated flag if more.

Files to modify:

• services/settlement-engine/src/index.ts — enhance reconcile handler
• shared/validation/schemas.ts

Test requirements:

1. Identical data — 0 mismatches.
2. Different status — appears in diff.
3. Missing in one system — appears as "missing".
4. Detail=false — count-only response.

Acceptance criteria:

• Operators can identify exact settlement mismatches.
• Detailed mode opt-in, capped at 100 records.