[NX-501 Issue] Add E2E API smoke test workflow and related test suite

This commit introduces a GitHub Actions workflow for running E2E API smoke tests on main branches and pull requests. It includes a test suite covering authentication, CRUD operations, metrics access, and alerts status. The README is updated with instructions for running the tests locally.

.github/workflows/e2e-api-smoke.yml

@@ -0,0 +1,78 @@
+name: E2E API Smoke
+
+on:
+  push:
+    branches: ["main", "master", "development"]
+    paths:
+      - "backend/**"
+      - ".github/workflows/e2e-api-smoke.yml"
+  pull_request:
+    paths:
+      - "backend/**"
+      - ".github/workflows/e2e-api-smoke.yml"
+  workflow_dispatch:
+
+jobs:
+  e2e-smoke:
+    name: Core API E2E Smoke
+    runs-on: ubuntu-latest
+
+    services:
+      postgres:
+        image: postgres:16
+        env:
+          POSTGRES_DB: nexapg
+          POSTGRES_USER: nexapg
+          POSTGRES_PASSWORD: nexapg
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U nexapg -d nexapg"
+          --health-interval 5s
+          --health-timeout 5s
+          --health-retries 20
+
+    env:
+      APP_NAME: NexaPG Monitor
+      ENVIRONMENT: test
+      LOG_LEVEL: INFO
+      DB_HOST: 127.0.0.1
+      DB_PORT: 5432
+      DB_NAME: nexapg
+      DB_USER: nexapg
+      DB_PASSWORD: nexapg
+      JWT_SECRET_KEY: smoke_jwt_secret_for_ci_only
+      JWT_ALGORITHM: HS256
+      JWT_ACCESS_TOKEN_MINUTES: 15
+      JWT_REFRESH_TOKEN_MINUTES: 10080
+      ENCRYPTION_KEY: 5fLf8HSTbEUeo1c4DnWnvkXxU6v8XJ8iW58wNw5vJ8s=
+      CORS_ORIGINS: http://localhost:5173
+      POLL_INTERVAL_SECONDS: 30
+      INIT_ADMIN_EMAIL: admin@example.com
+      INIT_ADMIN_PASSWORD: ChangeMe123!
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+
+      - name: Install backend dependencies + test tooling
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r backend/requirements.txt
+          pip install pytest
+
+      - name: Run Alembic migrations
+        working-directory: backend
+        run: alembic upgrade head
+
+      - name: Run core API smoke suite
+        env:
+          PYTHONPATH: backend
+        run: pytest -q backend/tests/e2e/test_api_smoke.py