feat: make invite link base URL configurable via APP_URL

Load environment variables from .env file using godotenv.
Defaults to https://demo.adriatica.io if APP_URL is not set.

main.go

@@ -9,6 +9,7 @@ import (
 	"os"
 
 	"github.com/google/uuid"
+	"github.com/joho/godotenv"
 
 	"github.com/ryanhamamura/c4/auth"
 	"github.com/ryanhamamura/c4/db"
@@ -46,6 +47,8 @@ func port() string {
 }
 
 func main() {
+	_ = godotenv.Load()
+
 	if err := db.Init("c4.db"); err != nil {
 		log.Fatal(err)
 	}