fix(relay): stop delivery attempts from growing the database forever (#26141)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: juliusmarminge

infra/relay/migrations/postgres/20261027210820_delivery_attempts_retention/migration.sql

@@ -0,0 +1,6 @@
+-- Alchemy runs migrations in a transaction, so CONCURRENTLY is unavailable here. Build
+-- idx_relay_delivery_attempts_created concurrently before deploying so this is a no-op.
+DROP INDEX IF EXISTS "idx_relay_delivery_attempts_environment";--> statement-breakpoint
+DROP INDEX IF EXISTS "idx_relay_agent_activity_rows_updated";--> statement-breakpoint
+DROP INDEX IF EXISTS "idx_relay_environment_credentials_environment";--> statement-breakpoint
+CREATE INDEX IF NOT EXISTS "idx_relay_delivery_attempts_created" ON "relay_delivery_attempts" ("created_at");