feat: auto-enable path-style addressing when a custom endpoint is set (#99)

## Summary

When `AWS_ENDPOINT_URL_S3` or `AWS_ENDPOINT_URL` is set — typically because the runtime is pointing at a local MinIO / S3-compatible endpoint — auto-enable path-style addressing on the S3 client. Without this, requests fail because MinIO does not implement virtual-hosted style addressing out of the box.

Production deployments leave those env vars unset and continue talking to real AWS S3 with virtual-hosted style — no behaviour change for prod.

Both `New()` and `NewS3()` share a `s3ClientOptions` helper that applies the toggle.

## Motivation

Spinning up a MinIO-backed acctest environment for Shiny (document-service, invoice-service, accounting-service). Without this change callers would have to sidestep `storage.New` and construct an `aws.Config` by hand just to flip `UsePathStyle`.

## Test plan

- [x] New unit test `TestS3ClientOptions_PathStyleTogglesOnCustomEndpoint` covers the three relevant env-var states
- [x] `go test ./...` passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #99

s3.go

@@ -3,6 +3,7 @@ package storage
 import (
 	"context"
 	"io"
+	"os"
 	"time"
 
 	"github.com/aws/aws-sdk-go-v2/aws"
@@ -12,6 +13,19 @@ import (
 	"github.com/aws/aws-sdk-go-v2/service/s3"
 )
 
+// s3ClientOptions returns the per-client overrides applied to every S3 client
+// constructed by this package. When the AWS_ENDPOINT_URL_S3 (or
+// AWS_ENDPOINT_URL) env var is set — typically because the runtime is
+// pointing at a local MinIO/S3-compatible endpoint — path-style addressing
+// is enabled so requests look like `http://host:9000/bucket/key` instead of
+// `http://bucket.host:9000/key`. Production deployments leave those vars
+// unset and continue talking to real S3 with virtual-hosted style.
+func s3ClientOptions(o *s3.Options) {
+	if os.Getenv("AWS_ENDPOINT_URL_S3") != "" || os.Getenv("AWS_ENDPOINT_URL") != "" {
+		o.UsePathStyle = true
+	}
+}
+
 // Uploader is the interface for uploading objects to S3 using the transfer manager
 type Uploader interface {
 	UploadObject(ctx context.Context, input *transfermanager.UploadObjectInput, opts ...func(*transfermanager.Options)) (*transfermanager.UploadObjectOutput, error)
@@ -131,7 +145,7 @@ func New(bucket string) (*S3, error) {
 	if err != nil {
 		return nil, err
 	}
-	client := s3.NewFromConfig(cfg)
+	client := s3.NewFromConfig(cfg, s3ClientOptions)
 	uploader := transfermanager.New(client, func(o *transfermanager.Options) {
 		o.PartSizeBytes = 5 * 1024 * 1024
 	})
@@ -147,7 +161,7 @@ func New(bucket string) (*S3, error) {
 // NewS3 creates a new S3 storage instance using direct PutObject
 // This is useful when you want more control over the AWS configuration
 func NewS3(cfg aws.Config, bucket string) *S3 {
-	client := s3.NewFromConfig(cfg)
+	client := s3.NewFromConfig(cfg, s3ClientOptions)
 	return &S3{
 		bucket:          bucket,
 		directSvc:       client,

s3_test.go

@@ -40,6 +40,35 @@ func (m *mockPresigner) PresignGetObject(ctx context.Context, params *s3.GetObje
 	return m.presignFunc(ctx, params, optFns...)
 }
 
+// Test path-style toggle
+
+func TestS3ClientOptions_PathStyleTogglesOnCustomEndpoint(t *testing.T) {
+	cases := []struct {
+		name     string
+		envVar   string
+		value    string
+		expected bool
+	}{
+		{name: "no env var → virtual-hosted", envVar: "", expected: false},
+		{name: "AWS_ENDPOINT_URL_S3 set → path-style", envVar: "AWS_ENDPOINT_URL_S3", value: "http://minio:9000", expected: true},
+		{name: "AWS_ENDPOINT_URL set → path-style", envVar: "AWS_ENDPOINT_URL", value: "http://minio:9000", expected: true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("AWS_ENDPOINT_URL", "")
+			t.Setenv("AWS_ENDPOINT_URL_S3", "")
+			if tc.envVar != "" {
+				t.Setenv(tc.envVar, tc.value)
+			}
+			opts := s3.Options{}
+			s3ClientOptions(&opts)
+			if opts.UsePathStyle != tc.expected {
+				t.Fatalf("UsePathStyle = %v, want %v", opts.UsePathStyle, tc.expected)
+			}
+		})
+	}
+}
+
 // Test NewS3 constructor
 
 func TestNewS3(t *testing.T) {