refactor: rename Batch to Partition across pipeline

Rename internal types and channels from Batch to Partition for consistency with the data model.

internal/app/config/main.go

@@ -8,10 +8,8 @@ import (
 )
 
 type appConfig struct {
-	SourceDbUrl  string
-	SourceDbType string
-	TargetDbUrl  string
-	TargetDbType string
+	SourceDbUrl string
+	TargetDbUrl string
 }
 
 func loadEnv() {
@@ -29,26 +27,14 @@ func getAppConfig() appConfig {
 		log.Fatal("SOURCE_DB_URL environment variable not set")
 	}
 
-	sourceDbType := os.Getenv("SOURCE_DB_TYPE")
-	if sourceDbType == "" {
-		log.Fatal("SOURCE_DB_TYPE environment variable not set")
-	}
-
 	targetDbUrl := os.Getenv("TARGET_DB_URL")
 	if targetDbUrl == "" {
 		log.Fatal("TARGET_DB_URL environment variable not set")
 	}
 
-	targetDbType := os.Getenv("TARGET_DB_TYPE")
-	if targetDbType == "" {
-		log.Fatal("TARGET_DB_TYPE environment variable not set")
-	}
-
 	return appConfig{
-		SourceDbUrl:  sourceDbUrl,
-		SourceDbType: sourceDbType,
-		TargetDbUrl:  targetDbUrl,
-		TargetDbType: targetDbType,
+		SourceDbUrl: sourceDbUrl,
+		TargetDbUrl: targetDbUrl,
 	}
 }