unite backend jobs into one workflow

.github/workflows/backend.yml

@@ -0,0 +1,108 @@
+name: Backend CI
+
+on:
+  push:
+    branches:
+      - master
+    paths:
+      - ".github/workflows/backend*"
+      - "backend/**"
+      - "!frontend/**"
+  pull_request:
+    paths:
+      - ".github/workflows/backend*"
+      - "backend/**"
+      - "!frontend/**"
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+env:
+  CARGO_TERM_COLOR: always
+
+defaults:
+  run:
+    working-directory: ./backend
+
+jobs:
+  check:
+    name: Check Code
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+
+      - name: Install Rust stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+
+      - name: Rust Cache
+        uses: Swatinem/rust-cache@23bce251a8cd2ffc3c1075eaa2367cf899916d84 # v2.7.3
+        with:
+          workspaces: backend
+
+      - name: Build
+        run: cargo check --all-targets --verbose
+  tests:
+    name: Run tests
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+
+      - name: Install Rust stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+
+      - name: Rust Cache
+        uses: Swatinem/rust-cache@23bce251a8cd2ffc3c1075eaa2367cf899916d84 # v2.7.3
+        with:
+          workspaces: backend
+
+      - name: Cargo test
+        run: cargo test --verbose --jobs 1
+  docs:
+    name: Check Documentation
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+
+      - name: Install Rust stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+
+      - name: Rust Cache
+        uses: Swatinem/rust-cache@23bce251a8cd2ffc3c1075eaa2367cf899916d84 # v2.7.3
+        with:
+          workspaces: backend
+
+      - name: Check internal documentation links
+        run: RUSTDOCFLAGS="--deny broken_intra_doc_links" cargo doc --verbose --workspace --no-deps --document-private-items
+  fmt:
+    name: Run rustfmt
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+
+      - name: Install Rust stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+          components: clippy, rustfmt
+
+      - name: Cargo fmt
+        run: cargo fmt --verbose --all -- --check