feat: Implement modular plugin architecture

- Convert invcount to self-contained module
- Add Module Manager for install/uninstall
- Create module_registry database table
- Support hot-reloading of modules
- Move data imports into invcount module
- Update all templates and routes to new structure

Version bumped to 0.16.0

templates/home.html

@@ -22,7 +22,7 @@
     {% if modules %}
         <div class="module-grid">
             {% for m in modules %}
-            <a href="{{ url_for(m.module_key + '.index') }}" class="module-card">
+            <a href="/{{ m.module_key }}" class="module-card">
                 <div class="module-icon">
                     <i class="fa-solid {{ m.icon }}"></i>
                 </div>