Make Square webhook dedup restart-safe via database

The in-memory dedup is now only a fast path; the square_webhook_events INSERT ... ON CONFLICT DO NOTHING is the source of truth, so replays across restarts and after FIFO eviction are skipped. A DB failure fails closed with 503 so Square retries. Empty event_ids are rejected with 400 (no dispatch, no dedup row). Ordering trade-off (insert-before-dispatch) documented for when handlers mutate state.

backend/handlers/webhooks/testmain_test.go

@@ -0,0 +1,20 @@
+//go:build test
+
+package webhooks
+
+import (
+	"os"
+	"testing"
+
+	"crussell/db"
+	"crussell/testutils/testdb"
+)
+
+func TestMain(m *testing.M) {
+	pool := testdb.CreateTestDatabase("crussell_test_handlers_webhooks")
+	db.Conn = db.NewPoolProxy(pool)
+	testdb.SeedBaseline(pool)
+	code := m.Run()
+	testdb.DestroyTestDatabase(pool, "crussell_test_handlers_webhooks")
+	os.Exit(code)
+}