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

AI Prompt.txt

@@ -31,7 +31,7 @@ Long-term goal: evolve into a WMS, but right now focus on making this workflow r
 ## Scanlook (current product summary)
 Scanlook is a web app for warehouse counting workflows built with Flask + SQLite.
 
-**Current Version:** 0.14.0
+**Current Version:** 0.15.0
 
 **Tech Stack:**
 - Backend: Python/Flask, raw SQL (no ORM), openpyxl (Excel file generation)
@@ -68,7 +68,7 @@ Scanlook is a web app for warehouse counting workflows built with Flask + SQLite
 
 **Long-term goal:** Modular WMS with future modules for Shipping, Receiving, Transfers, Production.
 
-**Module System (v0.14.0):**
+**Module System:**
 - Modules table defines available modules (module_key used for routing)
 - UserModules table tracks per-user access
 - Home page (/home) shows module cards based on user's access