Fix migration v4 ordering and add a v3->v4 regression test (AUD-70)

schema.sql runs before the incremental migrations, so the folder index must
not live there: on a v3 database the column does not exist yet and startup
failed with "no such column: folder". The migration still creates the index.
The regression test opens a hand-built v3 database with the current code.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

internal/store/triage_test.go

@@ -2,6 +2,7 @@ package store
 
 import (
 	"context"
+	"database/sql"
 	"path/filepath"
 	"testing"
 	"time"
@@ -34,6 +35,59 @@ func seedMessage(t *testing.T, s *Store, ctx context.Context, gmailID, subject,
 	return id
 }
 
+func TestMigrationFromV3AddsFolderColumns(t *testing.T) {
+	ctx := context.Background()
+	path := filepath.Join(t.TempDir(), "old.db")
+
+	// Simulate a database created by the v3 release: messages has the IMAP
+	// columns but not folder/mailbox. Opening it with the new code must apply
+	// migration v4 without failing on the folder index (the schema file is
+	// executed before migrations).
+	db, err := sql.Open("sqlite", "file:"+path)
+	if err != nil {
+		t.Fatalf("open old db: %v", err)
+	}
+	stmts := []string{
+		`CREATE TABLE messages (
+			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			gmail_id TEXT NOT NULL UNIQUE,
+			thread_id TEXT, from_addr TEXT, to_addr TEXT, subject TEXT,
+			body_text TEXT, source_address TEXT, date_sent TEXT,
+			imap_uid INTEGER, imap_uidvalidity INTEGER,
+			received_at TEXT NOT NULL DEFAULT (datetime('now')),
+			created_at TEXT NOT NULL DEFAULT (datetime('now')))`,
+		`CREATE TABLE schema_migrations (version INTEGER PRIMARY KEY, applied_at TEXT NOT NULL DEFAULT (datetime('now')))`,
+		`INSERT INTO schema_migrations (version) VALUES (3)`,
+	}
+	for _, q := range stmts {
+		if _, err := db.ExecContext(ctx, q); err != nil {
+			t.Fatalf("seed old db: %v", err)
+		}
+	}
+	_ = db.Close()
+
+	s, err := Open(ctx, path)
+	if err != nil {
+		t.Fatalf("Open() on v3 database error = %v", err)
+	}
+	defer s.Close()
+
+	v, err := s.SchemaVersion(ctx)
+	if err != nil {
+		t.Fatalf("SchemaVersion() error = %v", err)
+	}
+	if v < 4 {
+		t.Errorf("SchemaVersion() = %d, want >= 4", v)
+	}
+	var n int
+	if err := s.DB().QueryRowContext(ctx, `SELECT COUNT(*) FROM pragma_table_info('messages') WHERE name = 'folder'`).Scan(&n); err != nil {
+		t.Fatalf("count folder column: %v", err)
+	}
+	if n != 1 {
+		t.Errorf("folder column count = %d, want 1", n)
+	}
+}
+
 func TestMigrationAddsFolderColumns(t *testing.T) {
 	s, ctx := testStore(t)