feat: OAuth implementation with Discord

This commit separates the core features of GéJDR from the backend as
these will also be used by the bot in the future.

This commit also updates the dependencies of the project. It also
removes the dependency lettre as well as the mailpit docker service
for developers as it appears clearer this project won’t send emails
anytime soon.

The publication of a docker image is also postponed until later.

.gitea/workflows/publish.yaml.bak

@@ -0,0 +1,46 @@
+name: Publish Docker image
+on:
+  push:
+    branches:
+      - 'main'
+      - 'develop'
+    tags:
+      - 'v*'
+  pull_request:
+    branches:
+      - 'main'
+      - 'develop'
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Log in to Docker registry
+        uses: docker/login-action@v3.3.0
+        with:
+          username: ${{ secrets.DOCKER_REGISTRY_USERNAME }}
+          password: ${{ secrets.DOCKER_REGISTRY_PASSWORD }}
+          registry: ${{ var.REGISTRY }}
+      - uses: cachix/install-nix-action@v27
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - name: Build Docker image
+        run: nix develop --command -- just backend build-docker
+      - name: Load Docker image
+        run: docker load < result
+      - name: Docker Metadata action
+        uses: docker/metadata-action@v5.6.1
+        with:
+          image: gejdr-backend:latest
+          tags:
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha
+          labels: |
+            org.opencontainers.image.title=Backend GéJDR
+            org.opencontainers.image.description=Backend for GéJDR
+            org.opencontainers.image.vendor=Lucien Cartier-Tilet <lucien@phundrak.com>