Refactor content structure and add basic authentication utilities

- Moved event and gallery data to JSON files for cleaner content management.
- Added session management utilities with CSRF protection.
- Integrated OAuth-based login and logout APIs.
- Updated dependencies, including Astro and introduced dotenv-cli.
- Enhanced package.json with local environment support.

.env.example

@@ -0,0 +1,25 @@
+# Local development configuration for OAuth + Gitea
+# Copy this file to .env.local and fill in values, then run: npm run dev:local
+
+PUBLIC_BASE_URL=http://localhost:4321
+
+# Gitea OAuth app created with redirect URI: http://localhost:4321/api/auth/callback
+OAUTH_PROVIDER=gitea
+OAUTH_CLIENT_ID=
+OAUTH_CLIENT_SECRET=
+OAUTH_AUTHORIZE_URL=https://git.bookageek.ch/login/oauth/authorize
+OAUTH_TOKEN_URL=https://git.bookageek.ch/login/oauth/access_token
+OAUTH_USERINFO_URL=https://git.bookageek.ch/api/v1/user
+# Optional allow-list (comma separated usernames)
+# OAUTH_ALLOWED_USERS=
+
+# Gitea API for commits (service account PAT must have write:repository)
+GITEA_BASE=https://git.bookageek.ch
+GITEA_OWNER=
+GITEA_REPO=
+GITEA_TOKEN=
+GIT_BRANCH=main
+
+# Secrets (use long random strings)
+SESSION_SECRET=
+CSRF_SECRET=