refactor(pipeline): ввести Runner интерфейс

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

pkg/adapters/pipeline/pipeline.go

@@ -9,6 +9,18 @@ import (
 	"git.tswf.io/infra/go-synapse-backupper/pkg/domain/pgdump"
 )
 
+// Runner orchestrates the dump → encrypt → sink pipeline.
+type Runner interface {
+	// Run executes the full backup pipeline: pg_dump → encrypt → sink.
+	Run(
+		ctx context.Context,
+		pgDumpOpts pgdump.Options,
+		recipients []crypto.RecipientPub,
+		sink domain.Sink,
+		rand io.Reader,
+	) error
+}
+
 // Option configures a Runner.
 type Option func(*runner)
 
@@ -26,14 +38,14 @@ func WithEncryptor(encryptor crypto.Encryptor) Option {
 	}
 }
 
-// Runner orchestrates the dump → encrypt → sink pipeline.
+// runner is the private implementation of Runner.
 type runner struct {
 	dumper    pgdump.Dumper
 	encryptor crypto.Encryptor
 }
 
 // NewRunner creates a pipeline runner with the given functional options.
-func NewRunner(options ...Option) *runner {
+func NewRunner(options ...Option) Runner {
 	r := &runner{}
 	for _, option := range options {
 		option(r)