fix(mcp): change enable/disable endpoints from GET to POST and fix service/app listing

- `/mcp/enable` and `/mcp/disable` now use POST (state-mutating ops)
- `ListServices` queries DB directly instead of loading all projects into memory
- `ListApplications` validates tag arg rejects empty string (not just falsy)

openapi.yaml

@@ -5485,7 +5485,7 @@ paths:
         -
           bearerAuth: []
   /mcp/enable:
-    get:
+    post:
       summary: 'Enable MCP Server'
       description: 'Enable the MCP server endpoint at /mcp (only with root permissions).'
       operationId: enable-mcp
@@ -5514,7 +5514,7 @@ paths:
         -
           bearerAuth: []
   /mcp/disable:
-    get:
+    post:
       summary: 'Disable MCP Server'
       description: 'Disable the MCP server endpoint at /mcp (only with root permissions).'
       operationId: disable-mcp