ci: add backend test workflow

.github/workflows/backend.yml

@@ -0,0 +1,38 @@
+name: Backend CI
+on:
+  push:
+    branches: [main]
+    paths: ["backend/**", ".github/workflows/backend.yml"]
+  pull_request:
+    branches: [main]
+    paths: ["backend/**", ".github/workflows/backend.yml"]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: backend
+    services:
+      postgres:
+        image: postgres:16-alpine
+        env:
+          POSTGRES_USER: postgres
+          POSTGRES_PASSWORD: pass
+          POSTGRES_DB: loft_test
+        ports: ["5432:5432"]
+        options: >-
+          --health-cmd "pg_isready -U postgres"
+          --health-interval 5s --health-timeout 5s --health-retries 5
+    env:
+      DATABASE_URL: postgres://postgres:pass@localhost:5432/loft_test
+      STORAGE_PATH: /tmp/loft-files-test
+      SQLX_OFFLINE: "true"
+
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo install sqlx-cli --no-default-features --features postgres,rustls
+      - run: sqlx migrate run
+      - run: mkdir -p /tmp/loft-files-test
+      - run: cargo test