refactor(store): unify SQLite and Postgres into dialect-based SQLStore

Extract shared SQLStore with Dialect interface for the ~5% that
differs between backends (DDL, placeholders, sequence resets).

- New dialect.go: Dialect interface + placeholder rewriter (? → $N)
- New sqlstore.go: single implementation of all 19 Store methods
- sqlite.go: reduced from 286 to 83 lines (SQLiteDialect only)
- postgres.go: reduced from 266 to 78 lines (PostgresDialect only)
- main.go: use NewSQLiteStore/NewPostgresStore constructors

Zero CRUD logic duplication. Every future schema change written once.

cmd/goupkeep/main.go

@@ -80,16 +80,21 @@ func main() {
 	flag.Parse()
 
 	var s store.Store
+	var dbErr error
 	if *flagDBType == "postgres" {
-		s = &store.PostgresStore{ConnStr: *flagDSN}
+		s, dbErr = store.NewPostgresStore(*flagDSN)
 		fmt.Printf("Using PostgreSQL: %s\n", *flagDSN)
 	} else {
-		s = &store.SQLiteStore{DBPath: *flagDSN}
+		s, dbErr = store.NewSQLiteStore(*flagDSN)
 		fmt.Printf("Using SQLite: %s\n", *flagDSN)
 	}
+	if dbErr != nil {
+		fmt.Printf("Database connection error: %v\n", dbErr)
+		os.Exit(1)
+	}
 
 	if err := s.Init(); err != nil {
-		fmt.Printf("Database Init Error: %v\n", err)
+		fmt.Printf("Database init error: %v\n", err)
 		os.Exit(1)
 	}
 	store.SetGlobal(s)