feat(server): sniff sqlite panel restore uploads and keep the fallback on failure

The SQLite panel's Restore now detects the upload by content like the
PostgreSQL panel does: migration dumps are rebuilt with RestoreSQLite,
pg_dump archives get a clear error instead of 'Invalid db file format',
and every upload passes the panel-schema pre-flight before Xray stops.
The .backup fallback survives a failed Xray start and is named in the
error, the DB pool is reopened on every error path after CloseDB, and a
failed InitDB closes the imported file before restoring the fallback so
the rename cannot hit a Windows sharing violation.

internal/web/service/server_import_sniff_test.go

@@ -0,0 +1,45 @@
+package service
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/database"
+)
+
+func TestSniffImportKind(t *testing.T) {
+	cases := []struct {
+		name   string
+		header []byte
+		want   int
+	}{
+		{"pg custom archive", []byte("PGDMP\x01\x10\x04"), importKindPgDump},
+		{"raw sqlite database", []byte("SQLite format 3\x00rest of header"), importKindSQLiteDB},
+		{"sqlite cli dump without pragma", []byte("BEGIN TRANSACTION;\nCREATE TABLE t(i);"), importKindSQLiteDump},
+		{"bom and whitespace before pragma", []byte("\xef\xbb\xbf\r\n PRAGMA foreign_keys=OFF;"), importKindSQLiteDump},
+		{"plain-format postgres dump", []byte("--\n-- PostgreSQL database dump\n--"), importKindUnknown},
+		{"empty file", nil, importKindUnknown},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := sniffImportKind(tc.header); got != tc.want {
+				t.Errorf("sniffImportKind(%q) = %d, want %d", tc.header, got, tc.want)
+			}
+		})
+	}
+
+	t.Run("panel migration dump", func(t *testing.T) {
+		dbPath := filepath.Join(t.TempDir(), "x-ui.db")
+		if err := database.InitDB(dbPath); err != nil {
+			t.Fatalf("InitDB: %v", err)
+		}
+		t.Cleanup(func() { _ = database.CloseDB() })
+		dump, err := database.DumpSQLiteToBytes(dbPath)
+		if err != nil {
+			t.Fatalf("DumpSQLiteToBytes: %v", err)
+		}
+		if got := sniffImportKind(dump[:64]); got != importKindSQLiteDump {
+			t.Errorf("sniffImportKind(real migration dump) = %d, want %d", got, importKindSQLiteDump)
+		}
+	})
+}