fix(docker): add g++ for isotree build, add dashboard Dockerfile.tests

- bot-detector Dockerfile + Dockerfile.tests: install g++ for isotree C++ extension
- dashboard Dockerfile.tests: new smoke test (verify FastAPI app loads)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

services/dashboard/Dockerfile.tests

@@ -0,0 +1,8 @@
+FROM python:3.11-slim
+WORKDIR /app
+COPY services/dashboard/requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+RUN pip install --no-cache-dir pytest httpx
+COPY services/dashboard/backend/ ./backend/
+# Smoke test: verify the app imports and starts correctly
+CMD ["python", "-c", "from backend.main import app; print(f'FastAPI app loaded: {len(app.routes)} routes'); print('OK')"]