tighten backend regression coverage

Add targeted tests for rejection paths, settlement overrides, and key analytics flows, and align localization/research docs with the shipped contract.

docs/research/export-path.md

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+# Research Export Path
+
+## Goal
+
+Export study data without adding JSONB or ad hoc blobs.
+
+## Source Tables
+
+- `sessions`
+- `experiment_assignments`
+- `bet_intents`
+- `settlements`
+- `analytics_events`
+- `analytics_event_attributes`
+
+## Recommended Export Shape
+
+- `session_id`
+- `user_id`
+- `experiment_variant`
+- `locale_code`
+- `event_name`
+- `occurred_at`
+- `attribute_key`
+- `attribute_value`
+
+## Export Flow
+
+1. Join relational tables into a flat study dataset.
+2. Filter by session, date range, or experiment variant.
+3. Write CSV for quick review or Parquet for downstream analysis.
+4. Keep the export job read-only and repeatable.
+
+## Notes
+
+- Use the server clock for ordering and latency calculations.
+- Prefer one row per analytics event with child attribute expansion.
+- Keep locale and variant fields in every export.