feat: add logging and migration runner skeleton

Wire a structured logger and a top-level migration job that drives extract, transform and load stages.

internal/app/config/main.go

@@ -8,8 +8,10 @@ import (
 )
 
 type appConfig struct {
-	SourceDbUrl string
-	TargetDbUrl string
+	SourceDbUrl  string
+	SourceDbType string
+	TargetDbUrl  string
+	TargetDbType string
 }
 
 func loadEnv() {
@@ -27,14 +29,26 @@ 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,
-		TargetDbUrl: targetDbUrl,
+		SourceDbUrl:  sourceDbUrl,
+		SourceDbType: sourceDbType,
+		TargetDbUrl:  targetDbUrl,
+		TargetDbType: targetDbType,
 	}
 }