feat: init cloud calendar support (#14247)

#### PR Dependency Tree


* **PR #14247** 👈
  * **PR #14248**

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Google Calendar integration (disabled by default): link/unlink
accounts, OAuth flow, webhooks, real-time push, background sync,
workspace calendars with customizable items and date-range event
viewing.
* **GraphQL / Client**
* New queries & mutations for accounts, subscriptions, events,
providers, and workspace calendar management.
* **Localization**
* Added localized error message for calendar provider request failures.
* **Tests**
* Backend tests covering sync, webhook renewal, and error/error-recovery
scenarios.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.docker/selfhost/schema.json

@@ -650,6 +650,40 @@
         }
       }
     },
+    "calendar": {
+      "type": "object",
+      "description": "Configuration for calendar module",
+      "properties": {
+        "google": {
+          "type": "object",
+          "description": "Google Calendar integration config\n@default {\"enabled\":false,\"clientId\":\"\",\"clientSecret\":\"\",\"externalWebhookUrl\":\"\",\"webhookVerificationToken\":\"\"}\n@link https://developers.google.com/calendar/api/guides/push",
+          "properties": {
+            "enabled": {
+              "type": "boolean"
+            },
+            "clientId": {
+              "type": "string"
+            },
+            "clientSecret": {
+              "type": "string"
+            },
+            "externalWebhookUrl": {
+              "type": "string"
+            },
+            "webhookVerificationToken": {
+              "type": "string"
+            }
+          },
+          "default": {
+            "enabled": false,
+            "clientId": "",
+            "clientSecret": "",
+            "externalWebhookUrl": "",
+            "webhookVerificationToken": ""
+          }
+        }
+      }
+    },
     "captcha": {
       "type": "object",
       "description": "Configuration for captcha module",