feat(backend): initial setup for cms backend service

backend/.env.example

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+# Database (SQLite)
+DATABASE_PATH=./data/gallus_cms.db
+
+# Gitea OAuth
+GITEA_URL=https://git.bookageek.ch
+GITEA_CLIENT_ID=your-oauth-client-id-here
+GITEA_CLIENT_SECRET=your-oauth-client-secret-here
+GITEA_REDIRECT_URI=http://localhost:3000/api/auth/callback
+GITEA_ALLOWED_USERS=sabrina,raphael,admin
+
+# Git Configuration (use Gitea repository)
+GIT_REPO_URL=https://git.bookageek.ch/yourusername/Gallus_Pub.git
+GIT_TOKEN=your-gitea-personal-access-token-here
+GIT_USER_NAME=Gallus CMS
+GIT_USER_EMAIL=cms@galluspub.ch
+GIT_WORKSPACE_DIR=./data/workspace
+
+# JWT & Session
+JWT_SECRET=your-super-secret-jwt-key-change-this
+SESSION_SECRET=your-session-secret-change-this
+
+# Server
+PORT=3000
+NODE_ENV=development
+CORS_ORIGIN=http://localhost:5173
+FRONTEND_URL=http://localhost:5173
+
+# Upload
+MAX_FILE_SIZE=5242880