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

modules/invcount/manifest.json

@@ -0,0 +1,11 @@
+{
+  "module_key": "invcount",
+  "name": "Inventory Counts",
+  "version": "1.0.0",
+  "author": "STUFF",
+  "description": "Cycle counting and physical inventory workflows with session-based tracking",
+  "requires_roles": ["owner", "admin", "staff"],
+  "routes_prefix": "/invcount",
+  "has_migrations": true,
+  "dependencies": []
+}