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

app/plugins/builtins/mcp_server/plugin.py

@@ -0,0 +1,31 @@
+"""MCP Server plugin — exposes LeoCRM data to external MCP clients (Claude Desktop, etc.)."""
+
+from __future__ import annotations
+
+from app.plugins.base import BasePlugin
+from app.plugins.manifest import PluginManifest, PluginRouteDef
+
+
+class McpServerPlugin(BasePlugin):
+    """MCP Server plugin: exposes LeoCRM tools to external AI clients via MCP protocol."""
+
+    manifest = PluginManifest(
+        name="mcp_server",
+        version="1.0.0",
+        display_name="MCP Server",
+        description="Exposes LeoCRM data (contacts, calendar, mail, DMS) to external MCP clients via API-token auth.",
+        dependencies=["permissions"],
+        routes=[
+            PluginRouteDef(
+                path="/api/v1/mcp",
+                module="app.plugins.builtins.mcp_server.routes",
+                router_attr="router",
+            ),
+        ],
+        events=[],
+        migrations=["0001_initial.sql"],
+        permissions=[
+            "mcp:read",
+            "mcp:write",
+        ],
+    )