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>

config.s3.json.sample

@@ -0,0 +1,20 @@
+{
+  "weburl": "https://yourapp.com",
+  "address": "localhost", 
+  "port": 8080,
+  "apiprefix": "/api/v1",
+  "databasedriver": "sqlite",
+  "databasefile": "./openaccounting.db",
+  "storage": {
+    "backend": "s3",
+    "s3": {
+      "region": "us-east-1",
+      "bucket": "my-openaccounting-attachments",
+      "prefix": "attachments",
+      "access_key_id": "",
+      "secret_access_key": "",
+      "endpoint": "",
+      "path_style": false
+    }
+  }
+}