feat: MCP Server plugin (Task 5.16) - expose LeoCRM tools to external AI clients

- New plugin app/plugins/builtins/mcp_server/ with 9 MCP tools
- Tools: search_contacts, get_contact, create_contact, list_calendar_entries,
  create_calendar_entry, list_emails, send_email, list_files, upload_file
- Routes: GET /api/v1/mcp/tools, POST /api/v1/mcp/tools/{name}/execute, GET /api/v1/mcp/config
- API-token auth via session + RBAC (mcp:read, mcp:write)
- Frontend: mcp.ts API client with React Query hooks
- Frontend: SettingsMcp.tsx settings page with tool listing and execution
- i18n: de.json and en.json updated with MCP entries
- Tests: 7 tests covering tool listing, config, execution, auth, schema validation

frontend/src/i18n/locales/en.json

@@ -16,7 +16,8 @@
     "users": "Users",
     "auditLog": "Audit Log",
     "settings": "Settings",
-    "aiAssistant": "AI Assistant"
+    "aiAssistant": "AI Assistant",
+    "mcpSettings": "MCP Settings"
   },
   "auth": {
     "login": "Sign In",
@@ -284,7 +285,8 @@
     "livePreview": "Live Preview",
     "livePreviewDescription": "This is how the app looks with the current theme",
     "resetTheme": "Reset",
-    "saveTheme": "Save theme"
+    "saveTheme": "Save theme",
+    "mcp": "MCP"
   },
   "auditLog": {
     "title": "Audit Log",
@@ -918,5 +920,52 @@
       "tab": "Switch tab",
       "settings": "Change settings"
     }
+  },
+  "mcp": {
+    "title": "MCP Server & Client",
+    "server": {
+      "title": "MCP Server",
+      "description": "LeoCRM as MCP Server for external AI clients",
+      "tools": "Available Tools",
+      "toolCount": "Tool Count",
+      "toolName": "Tool",
+      "toolDescription": "Description",
+      "toolCategory": "Category",
+      "toolPermission": "Permission",
+      "execute": "Execute",
+      "executeTool": "Execute Tool",
+      "arguments": "Arguments (JSON)",
+      "result": "Result",
+      "error": "Error",
+      "success": "Success",
+      "config": "Server Configuration",
+      "serverName": "Server Name",
+      "serverVersion": "Version",
+      "protocolVersion": "Protocol Version",
+      "authMethod": "Authentication",
+      "availableTools": "Available Tools",
+      "noTools": "No tools available"
+    },
+    "client": {
+      "title": "MCP Client",
+      "description": "Description",
+      "servers": "Server Configurations",
+      "addServer": "Add Server",
+      "editServer": "Edit Server",
+      "deleteServer": "Delete Server",
+      "serverName": "Name",
+      "serverUrl": "URL",
+      "apiToken": "API Token",
+      "enabled": "Enabled",
+      "lastConnected": "Last Connected",
+      "noServers": "No servers configured",
+      "deleteConfirm": "Are you sure you want to delete this server?",
+      "serverCreated": "Server created successfully.",
+      "serverUpdated": "Server updated successfully.",
+      "serverDeleted": "Server deleted successfully.",
+      "viewTools": "View Tools",
+      "tools": "Tools",
+      "noTools": "No tools available"
+    }
   }
-}
+}