fix: recreate ops_alert_silences table dropped by goose-style migration

backend/migrations/071_recreate_ops_alert_silences.sql

@@ -0,0 +1,27 @@
+-- Recreate ops_alert_silences table.
+--
+-- Migration 037 used goose-style annotations (-- +goose Up / -- +goose Down),
+-- but the project's migration runner does not parse goose directives.
+-- It executed the entire file as plain SQL, including the Down section
+-- (DROP TABLE IF EXISTS ops_alert_silences), which immediately destroyed
+-- the table that the Up section had just created.
+--
+-- This migration recreates the table using IF NOT EXISTS for idempotency.
+
+CREATE TABLE IF NOT EXISTS ops_alert_silences (
+    id BIGSERIAL PRIMARY KEY,
+
+    rule_id BIGINT NOT NULL,
+    platform VARCHAR(64) NOT NULL,
+    group_id BIGINT,
+    region VARCHAR(64),
+
+    until TIMESTAMPTZ NOT NULL,
+    reason TEXT,
+
+    created_by BIGINT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_ops_alert_silences_lookup
+    ON ops_alert_silences (rule_id, platform, group_id, region, until);