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.b2.json.sample

@@ -0,0 +1,17 @@
+{
+  "weburl": "https://yourapp.com",
+  "address": "localhost",
+  "port": 8080, 
+  "apiprefix": "/api/v1",
+  "databasedriver": "sqlite",
+  "databasefile": "./openaccounting.db",
+  "storage": {
+    "backend": "b2",
+    "b2": {
+      "account_id": "your-b2-account-id",
+      "application_key": "your-b2-application-key", 
+      "bucket": "my-openaccounting-attachments",
+      "prefix": "attachments"
+    }
+  }
+}