deps: update vendor dependencies for S3-compatible storage

Updates AWS SDK and removes Blazer B2 dependency in favor of unified
S3-compatible approach. Includes configuration examples and documentation.

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

Co-Authored-By: Claude <noreply@anthropic.com>

.env.storage.example

@@ -0,0 +1,38 @@
+# OpenAccounting Storage Configuration
+# Copy this file to .env and modify as needed
+
+# Database Configuration
+OA_DATABASEDRIVER=sqlite
+OA_DATABASEFILE=./openaccounting.db
+OA_ADDRESS=localhost
+OA_PORT=8080
+OA_APIPREFIX=/api/v1
+
+# Storage Backend Configuration
+# Options: local, s3, b2
+OA_STORAGE_BACKEND=local
+
+# Local Storage Configuration
+OA_STORAGE_LOCAL_ROOTDIR=./uploads
+OA_STORAGE_LOCAL_BASEURL=
+
+# Amazon S3 Storage Configuration (uncomment if using S3)
+# OA_STORAGE_S3_REGION=us-east-1
+# OA_STORAGE_S3_BUCKET=my-openaccounting-attachments
+# OA_STORAGE_S3_PREFIX=attachments
+# OA_STORAGE_S3_ACCESSKEYID=AKIA...
+# OA_STORAGE_S3_SECRETACCESSKEY=...
+# OA_STORAGE_S3_ENDPOINT=
+# OA_STORAGE_S3_PATHSTYLE=false
+
+# Backblaze B2 Storage Configuration (uncomment if using B2)
+# OA_STORAGE_B2_ACCOUNTID=your-b2-account-id
+# OA_STORAGE_B2_APPLICATIONKEY=your-b2-application-key
+# OA_STORAGE_B2_BUCKET=my-openaccounting-attachments
+# OA_STORAGE_B2_PREFIX=attachments
+
+# Email Configuration (optional)
+# OA_MAILGUNDOMAIN=
+# OA_MAILGUNKEY=
+# OA_MAILGUNEMAIL=
+# OA_MAILGUNSENDER=