fix: make version display dynamic across all surfaces

- routes.py: fallback reads from importlib.metadata instead of hardcoded
- login.html: remove hardcoded default version
- README: use dynamic GitHub Release badge from shields.io
- DOCKER_README: remove hardcoded version string

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/templates/dashboard/login.html

@@ -139,7 +139,7 @@
 
         <!-- Footer -->
         <p class="text-center text-gray-500 text-sm mt-6">
-            MCP Hub v{{ version|default('3.0.1') }}
+            MCP Hub v{{ version }}
         </p>
     </div>
 </body>