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_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) {