feat: Apple CalDAV credentials form + connect/disconnect UI (BL-04)

Admin can now enter CalDAV URL, Apple-ID and app-specific password
directly in Settings; credentials are tested live before saving and
stored in sync_config (take precedence over .env); disconnect clears
DB-stored credentials without server restart. Auto-sync interval
(15 min, configurable via SYNC_INTERVAL_MINUTES) was already in place.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Budget: recurring entries auto-generate instances for each viewed month; instances deleted by the user are skipped permanently via `budget_recurrence_skipped` table; generated instances are marked with ↩ in the transaction list
 - Budget: month-over-month comparison in summary cards — each card (Einnahmen, Ausgaben, Saldo) shows a trend line (▲/▼ + delta amount vs. previous month); previous month summary is fetched in parallel with current month
 - Meals: drag & drop between slots and days using Pointer Events (touch + mouse); ghost element follows pointer; drop on occupied slot swaps meals; reduced-motion: no ghost animation, interaction still works
+- Settings: Apple CalDAV credentials form (URL, Apple-ID, app-specific password) with live connection test; admin can connect and disconnect via UI without restarting the server; DB-stored credentials take precedence over .env vars; auto-sync runs every 15 min (configurable via SYNC_INTERVAL_MINUTES)
 
 ## [0.2.1] - 2026-03-30