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.

internal/store/dialect.go

@@ -0,0 +1,36 @@
+package store
+
+import "database/sql"
+
+type Dialect interface {
+	DriverName() string
+	CreateTablesSQL() []string
+	MigrationsSQL() []string
+	BoolFalse() string
+	ResetSequenceOnEmpty(db *sql.DB, table string)
+	ImportWipe(tx *sql.Tx)
+	ImportResetSequences(tx *sql.Tx)
+}
+
+// rewritePlaceholders converts ? markers to $1, $2, etc. for Postgres.
+// For SQLite (or any dialect not needing rewrite), returns the input unchanged.
+func rewritePlaceholders(query string, dollarStyle bool) string {
+	if !dollarStyle {
+		return query
+	}
+	buf := make([]byte, 0, len(query)+32)
+	n := 0
+	for i := 0; i < len(query); i++ {
+		if query[i] == '?' {
+			n++
+			buf = append(buf, '$')
+			if n >= 10 {
+				buf = append(buf, byte('0'+n/10))
+			}
+			buf = append(buf, byte('0'+n%10))
+		} else {
+			buf = append(buf, query[i])
+		}
+	}
+	return string(buf)
+}