v0.55.17

Changed

• Documentation synced with changelog v0.45.0–v0.55.16: SPEC.md adds oidc_sub/oidc_provider columns to the Users table and documents the SSO login flow (Authorization Code + PKCE, nonce) and failed-login warning logging; docs/installation.md adds TRUST_PROXY to the Server section, DATA_DIR/BACKUP_DIR to the Database section, a new SSO/OIDC env-var section, a new Automated Backups env-var section, and a legacy note on the Apple CalDAV single-account variables; README.md adds SSO/OpenID Connect to the Design & Technology section; docs/index.html updates the version badge and footer to v0.55.16 and corrects the birthday feature description to mention customizable reminders; docs/install.html updates the Calendar Sync optional card to reflect multi-account CalDAV/CardDAV, adds an SSO/OIDC optional card, updates the Automated Backups card to describe the built-in scheduler, and keeps EN and DE i18n strings in sync.