feat: rewrite the backend in Rust

Replaces the Go server with a Rust one (axum, tokio, sqlx, feed-rs,
ammonia, reqwest) serving the same REST/WebSocket API, so the React
frontend is unchanged.

- Same database: migrations keep the schema_migrations table and file
  names, Argon2id hashes and hashed session tokens created by the Go
  server keep working (users stay logged in), and feed item GUIDs are
  computed the same way, so existing articles are not re-ingested.
- Same security model: SSRF-safe fetching (public addresses only,
  checked at resolution time and on every redirect), HTML sanitization,
  CSP and security headers, same-origin guard, WebSocket origin check,
  per-IP / per-user rate limits, trusted-proxy aware client IPs.
- Fix: articles whose publication date lies in the future (ingested
  before the clamp existed) pinned the top of the list. The maintenance
  job now pulls such dates back at startup and daily.
- Relative links and images in feed content are resolved against the
  article URL instead of being dropped.
- Deploy: static musl binary in an Alpine image; the Unraid compose file
  uses pull_policy: always so the new image is applied on the next
  `docker compose up -d`. CI runs fmt, clippy and tests (including a
  PostgreSQL integration test) before building the image.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fc1UFpB4V1MGWLxHAu3BMK

.github/workflows/docker-publish.yml

@@ -11,7 +11,44 @@ env:
   IMAGE_NAME: ${{ github.repository }}
 
 jobs:
+  test:
+    runs-on: ubuntu-latest
+    services:
+      postgres:
+        image: postgres:16-alpine
+        env:
+          POSTGRES_USER: flowreader
+          POSTGRES_PASSWORD: flowreader
+          POSTGRES_DB: flowreader
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U flowreader" --health-interval 5s --health-timeout 5s --health-retries 10
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          components: clippy, rustfmt
+
+      - name: Cache cargo
+        uses: Swatinem/rust-cache@v2
+
+      - name: Format
+        run: cargo fmt --check
+
+      - name: Clippy
+        run: cargo clippy --all-targets --locked -- -D warnings
+
+      - name: Tests
+        env:
+          TEST_DATABASE_URL: postgres://flowreader:flowreader@localhost:5432/flowreader?sslmode=disable
+        run: cargo test --locked
+
   build:
+    needs: test
     runs-on: ubuntu-latest
     permissions:
       contents: read