tests: add backend tests

backend/AGENTS.md

@@ -16,6 +16,23 @@ golangci-lint run
 go test ./...
 ```
 
+## Testing
+
+All Go code is tested using the [testify](https://github.com/stretchr/testify) framework with comprehensive test coverage:
+
+- **Unit Tests**: Each function and method is thoroughly tested including edge cases
+- **Integration Tests**: Store implementations are tested end-to-end 
+- **Error Handling**: All error conditions are explicitly tested
+- **Concurrency**: Thread-safety of concurrent operations is verified
+- **File Operations**: File-based storage tests use temporary directories
+
+Tests follow these conventions:
+- Use `require.NoError(t, err)` for fatal errors that break test flow
+- Use `assert.NoError(t, err)` for non-fatal assertions  
+- Test both success and error paths for all methods
+- Use table-driven tests where appropriate
+- All tests are run with `go test ./...` or `go test -v ./...`
+
 ## Non-Negotiables
 
 - ❌ No unhandled errors - always check `err`
@@ -40,7 +57,7 @@ go test ./...
 - **DI**: Dependencies through constructors (`New*` functions)
 - **HTTP**: Handlers receive `store.Store`, validate inputs, return proper status codes
 - **Streaming**: Use `FlushWriter` for SSE/text streams
-- **Storage**: JSON file-based (`FileStore` implementation)
+- **Storage**: JSON file-based (`FileStore` implementation) with in-memory alternative (`InMemoryStore`)
 
 ## What Goes Where