docs: add calendar integration spec

docs/TASKS/calendar-integrations/001-backend-calendar-source-foundation.md

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+# Task: Backend calendar source foundation
+
+## Goal
+
+Add backend storage and APIs for organization, workspace, and user calendar sources.
+
+## Feature Spec
+
+- `docs/FEATURES/calendar-integrations.md`
+
+## Scope
+
+- Add a calendar integrations module under `backend/src/Socialize.Api/Modules/CalendarIntegrations`.
+- Model calendar sources for organization, workspace, and user scopes.
+- Support organization inheritance modes: required and optional.
+- Track source URL, catalog reference, display title, color, category, enabled state, and sync metadata.
+- Add endpoints to list visible sources for a workspace/user context.
+- Add endpoints to create/update/delete sources according to scope permissions.
+- Show inherited organization sources in workspace responses as read-only.
+- Add validation for source URLs, colors, scope, and inheritance mode.
+- Add focused backend tests for permissions and inherited source visibility.
+
+## Relevant Files
+
+- `backend/src/Socialize.Api/Modules/CalendarIntegrations/`
+- `backend/src/Socialize.Api/Data/AppDbContext.cs`
+- `backend/src/Socialize.Api/Program.cs`
+- `backend/tests/Socialize.Tests/`
+- `shared/openapi/openapi.json`
+- `frontend/src/api/schema.d.ts`
+
+## Validation
+
+```bash
+dotnet build backend/Socialize.slnx
+dotnet test backend/Socialize.slnx
+./scripts/update-openapi.sh
+```