test: add postgres idempotency store integration coverage

[devfoma]

Summary

• add build-tagged Postgres testcontainer coverage for PostgresIdempotencyStore
• cover concurrent GetOrInsert, TTL reuse, payload hash mismatch, and cancelled-context rollback behavior
• apply only the idempotency table migration inside the test setup

Closes #771

Tests

• gofmt -w internal/middleware/idempotency_store_integration_test.go
• go test ./internal/middleware (blocked: upstream go.sum is missing github.com/klauspost/compress@v1.15.9 go.mod entry required through github.com/segmentio/kafka-go)

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