Refactor webpush store to eliminate code duplication

Consolidate SQLite and Postgres store implementations into a single
commonStore with database-specific SQL queries passed via configuration.
This eliminates ~100 lines of duplicate code while maintaining full
functionality for both backends.

Key changes:
- Move all store methods to commonStore in store.go
- Remove sqliteStore and postgresStore wrapper structs
- Refactor SQLite to use QueryRow() pattern like Postgres
- Pass database-specific queries via storeQueries struct
- Make store types unexported, only expose Store interface

All tests pass for both SQLite and PostgreSQL backends.

webpush/store_sqlite_test.go

@@ -8,7 +8,7 @@ import (
 	"heckel.io/ntfy/v2/webpush"
 )
 
-func newTestSQLiteStore(t *testing.T) *webpush.SQLiteStore {
+func newTestSQLiteStore(t *testing.T) webpush.Store {
 	store, err := webpush.NewSQLiteStore(filepath.Join(t.TempDir(), "webpush.db"), "")
 	require.Nil(t, err)
 	t.Cleanup(func() { store.Close() })