fix(tradein): 030 — UNIQUE constraint on avito_imv_evaluations.cache_key (unblocks PR #452) (#454)

Hotfix unblocking PR #452 (Stage 3 IMV estimator integration).

Adds 030_avito_imv_cache_key_unique.sql: creates UNIQUE INDEX imv_cache_key_uniq_idx
on avito_imv_evaluations(cache_key), allowing save_imv_evaluation's ON CONFLICT (cache_key)
DO UPDATE to resolve. Migration 018 only created a non-unique composite index, which would
have caused runtime error on first IMV cache write.

DELETE-USING dedup guards against historical duplicates (no-op on fresh table).
BEGIN/COMMIT atomic; IF NOT EXISTS idempotent; CONCURRENTLY unnecessary (table empty).

Lexicographic apply order with PR #453's 030_listings_alter_yandex.sql is safe — different
tables, no functional interaction.

Reviewed by deep-code-reviewer: APPROVE.

tradein-mvp/backend/data/sql/030_avito_imv_cache_key_unique.sql

@@ -0,0 +1,26 @@
+-- 030_avito_imv_cache_key_unique.sql
+-- Hotfix: добавить UNIQUE constraint на avito_imv_evaluations.cache_key
+--
+-- Stage 1 миграция 018 создала только non-unique `imv_cache_key_idx` —
+-- но `save_imv_evaluation` использует `ON CONFLICT (cache_key) DO UPDATE`,
+-- что требует UNIQUE/exclusion constraint, иначе runtime error:
+--   ERROR: there is no unique or exclusion constraint matching the ON CONFLICT specification
+--
+-- Закрывает blocker reviewer'а на PR #452 (Stage 3 IMV estimator integration).
+--
+-- Apply after: 018_avito_imv_evaluations.sql (Stage 1)
+-- Required by: estimator IMV cache save flow (Stage 3, PR #452)
+
+BEGIN;
+
+-- Защитная очистка дубликатов по cache_key (если данные уже наливались).
+-- На свежей БД (cache_key никогда не writev до этого PR) — no-op.
+DELETE FROM avito_imv_evaluations a
+USING avito_imv_evaluations b
+WHERE a.id < b.id AND a.cache_key = b.cache_key;
+
+-- Сам UNIQUE constraint через индекс (CONCURRENTLY не нужен — таблица свежая/пустая).
+CREATE UNIQUE INDEX IF NOT EXISTS imv_cache_key_uniq_idx
+    ON avito_imv_evaluations (cache_key);
+
+COMMIT;