add architecture documentation and testing strategy

Co-authored-by: Copilot <copilot@github.com>

docs/TESTING.md

@@ -0,0 +1,19 @@
+# Testing Strategy
+
+The FastAPI backend will be tested using a combination of unit and integration tests.
+
+## Unit Tests
+
+- Use `pytest` with `pytest-asyncio` for async endpoint handlers.
+- Mock external dependencies such as the DuckDB connection and the openrouter.ai API.
+
+## Integration Tests
+
+- Use `pytest-flask` to spin up the FastAPI app in a test client.
+- Test full request/response cycles for authentication, user CRUD, and AI generation endpoints.
+- Use an in‑memory DuckDB instance for database tests.
+
+## Test Coverage
+
+- Aim for >80 % coverage on the `app/` package.
+- Generate coverage reports with `pytest-cov`.