feat: basic caldav support (#14372)

fix #13531 

#### PR Dependency Tree


* **PR #14372** 👈

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**
* CalDAV calendar integration: link and sync CalDAV-compatible calendars
(discovery, listing, event sync).
* New UI flow and dialog to link CalDAV accounts with provider
selection, credentials, and display name.

* **API / Config**
* Server exposes CalDAV provider presets in config and new GraphQL
mutation to link CalDAV accounts.
  * New calendar config section for CalDAV with validation and defaults.

* **Tests**
  * Comprehensive CalDAV integration test suite added.

* **Chores**
* Removed analytics tokens from build configuration and reduced Cloud
E2E test shards.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.docker/selfhost/schema.json

@@ -868,6 +868,72 @@
             "externalWebhookUrl": "",
             "webhookVerificationToken": ""
           }
+        },
+        "caldav": {
+          "type": "object",
+          "description": "CalDAV integration config\n@default {\"enabled\":false,\"allowCustomProvider\":false,\"providers\":[],\"allowInsecureHttp\":false,\"allowedHosts\":[],\"blockPrivateNetwork\":true,\"requestTimeoutMs\":10000,\"maxRedirects\":5}",
+          "properties": {
+            "enabled": {
+              "type": "boolean"
+            },
+            "allowCustomProvider": {
+              "type": "boolean"
+            },
+            "providers": {
+              "type": "array",
+              "items": {
+                "type": "object",
+                "properties": {
+                  "id": {
+                    "type": "string"
+                  },
+                  "label": {
+                    "type": "string"
+                  },
+                  "serverUrl": {
+                    "type": "string"
+                  },
+                  "authType": {
+                    "type": "string"
+                  },
+                  "requiresAppPassword": {
+                    "type": "boolean"
+                  },
+                  "docsUrl": {
+                    "type": "string"
+                  }
+                }
+              }
+            },
+            "allowInsecureHttp": {
+              "type": "boolean"
+            },
+            "allowedHosts": {
+              "type": "array",
+              "items": {
+                "type": "string"
+              }
+            },
+            "blockPrivateNetwork": {
+              "type": "boolean"
+            },
+            "requestTimeoutMs": {
+              "type": "number"
+            },
+            "maxRedirects": {
+              "type": "number"
+            }
+          },
+          "default": {
+            "enabled": false,
+            "allowCustomProvider": false,
+            "providers": [],
+            "allowInsecureHttp": false,
+            "allowedHosts": [],
+            "blockPrivateNetwork": true,
+            "requestTimeoutMs": 10000,
+            "maxRedirects": 5
+          }
         }
       }
     },