Add design spec for Nextcloud CalDAV calendar sync

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

docs/superpowers/specs/2026-04-10-nextcloud-calendar-sync-design.md

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+# Nextcloud Calendar Sync — Design Spec
+
+**Date:** 2026-04-10  
+**Status:** Approved
+
+---
+
+## Overview
+
+Add a second calendar sync target (Nextcloud CalDAV) alongside the existing Google Calendar sync. Shared logic is extracted into a common interface and a static helper, keeping both service implementations independent and self-contained.
+
+---
+
+## Architecture
+
+### Shared interface: `ICalendarService`
+
+Replaces `IGoogleCalendarService`. Both providers implement this interface:
+
+```csharp
+public interface ICalendarService
+{
+    Task<List<CalendarInfo>> GetCalendarsAsync();
+    Task SyncShiftsToCalendarAsync(string calendarId, List<ShiftWithTransport> shifts);
+}
+
+public class CalendarInfo
+{
+    public string Id { get; set; }
+    public string Summary { get; set; }
+}
+```
+
+### Shared helper: `CalendarEventBuilder` (static class)
+
+Extracts pure data-transformation logic currently inside `GoogleCalendarService`:
+
+- `FormatTransportInfo(TransportConnection)` → string
+- `BuildEventContent(Shift, DateTimeOffset start, DateTimeOffset end, TransportConnection?)` → returns a shared `CalendarEventData` record (summary + description)
+
+No orchestration logic lives here — just pure functions with no side effects.
+
+### `GoogleCalendarService`
+
+- Implements `ICalendarService`
+- Refactored to use `CalendarEventBuilder` for event content
+- All Google client library calls remain here
+- `GetCalendarsAsync()` returns `List<CalendarInfo>` mapped from Google's `CalendarListEntry`
+- Identification of owned events via `ExtendedProperties.Private["shiftSchedulerEvent"]` unchanged
+
+### `NextcloudCalendarService`
+
+- Implements `ICalendarService`
+- Uses `HttpClient` with Basic Auth (username + app password from configuration)
+- CalDAV operations:
+  - **List calendars**: `PROPFIND` on `/remote.php/dav/calendars/<username>/`, depth 1, parse `displayname` and `resourcetype`
+  - **Delete existing events in range**: `REPORT` (CalDAV `calendar-query`) to find VEVENTs with `X-SHIFT-SCHEDULER:true` property in the date range, then `DELETE` each by URL
+  - **Create events**: `PUT` a VEVENT (iCalendar format using the existing `Ical.Net` dependency) to `<calendar-url>/<uid>.ics`. Each event includes `X-SHIFT-SCHEDULER:true` as a custom property for future identification.
+- UIDs are generated deterministically: `shift-<shiftName>-<dateISO>-<morning|afternoon>` to allow idempotent re-sync if needed
+- Retry logic mirrors Google's exponential backoff, extracted from `GoogleCalendarService` into a shared `RetryHelper` static method in `CalendarEventBuilder`
+
+---
+
+## Controller
+
+`ShiftController` is updated:
+
+- Both `IGoogleCalendarService` and `INextcloudCalendarService` injected. These are empty marker interfaces that extend `ICalendarService`, registered with their concrete types in DI. This avoids DI ambiguity while keeping the shared interface as the contract:
+  ```csharp
+  public interface IGoogleCalendarService : ICalendarService { }
+  public interface INextcloudCalendarService : ICalendarService { }
+  ```
+- Old endpoints `GET /api/shift/google_calendars` and `POST /api/shift/sync_to_google_calendar` are **replaced** by:
+  - `GET /api/shift/calendars?provider=google|nextcloud` → `List<CalendarInfo>`
+  - `POST /api/shift/sync_to_calendar` → body: `{ Provider, CalendarId, Year, Month }`
+- Provider routing via a `switch` on the `provider` string — no factory abstraction
+
+---
+
+## Client UI
+
+### Buttons
+
+Two separate sync buttons in `Index.razor`:
+- **Sync to Google Calendar** (existing, unchanged label)
+- **Sync to Nextcloud** (new)
+
+Both buttons are disabled while `_isSyncing` is true.
+
+### State
+
+New field `_syncProvider` (`"google"` or `"nextcloud"`) tracks which provider triggered the current flow.
+
+Existing `_availableCalendars`, `_showCalendarSelector`, and the calendar picker modal are reused for both providers. The modal title changes to "Select Google Calendar" or "Select Nextcloud Calendar" based on `_syncProvider`.
+
+### Client DTO
+
+`GoogleCalendar` record on the client is renamed to `CalendarInfo` to match the server DTO.
+
+---
+
+## Configuration
+
+### `appsettings.json` (placeholder — committed to git)
+
+```json
+"Nextcloud": {
+  "BaseUrl": "https://your-nextcloud-instance.example.com",
+  "Username": "YOUR_NEXTCLOUD_USERNAME",
+  "AppPassword": "YOUR_NEXTCLOUD_APP_PASSWORD"
+}
+```
+
+### Production appsettings (not committed, mounted via Docker)
+
+Contains real values for `BaseUrl`, `Username`, and `AppPassword`.
+
+Read in `Program.cs` via `IConfiguration`, injected into `NextcloudCalendarService` — same pattern as `Authentication:Google`.
+
+### `DOCKER_CONFIG.md`
+
+Add a note that the `Nextcloud` section must be populated in the production `appsettings.json`.
+
+---
+
+## Dependencies
+
+No new NuGet packages required:
+- CalDAV HTTP calls: plain `HttpClient`
+- iCalendar VEVENT generation: `Ical.Net` (already in `ShiftScheduler.Services.csproj`)
+- XML parsing for CalDAV responses: `System.Xml` (built-in)
+
+---
+
+## Out of scope
+
+- Two-way sync (read events from calendar back into the scheduler)
+- Nextcloud OAuth — app password is sufficient for a personal instance
+- Caching of calendar lists