Add PC uptime tracking feature

- Database: Add lastboottime column to machines table
- API: Accept lastBootUpTime parameter and store in lastboottime column
- PowerShell: Collect LastBootUpTime from Win32_OperatingSystem
  - Update-PC-Minimal.ps1: Add last boot time collection
  - Update-ShopfloorPCs-Remote.ps1: Add last boot time collection and API posting
- Display: Add Uptime column to displaypcs.asp with color-coded badges
  - > 90 days: red badge
  - > 30 days: yellow badge
  - > 7 days: blue badge
  - <= 7 days: muted text
- Filter: Add "Uptime > X days" filter dropdown (7, 30, 90 days)
- SQL: Production migration script for lastboottime column

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

sql/add_lastboottime_column.sql

@@ -0,0 +1,29 @@
+-- Add lastboottime column to machines table for PC uptime tracking
+-- Run on production database
+-- Date: 2025-12-09
+
+-- Check if column exists before adding
+SET @dbname = DATABASE();
+SET @tablename = 'machines';
+SET @columnname = 'lastboottime';
+SET @preparedStatement = (SELECT IF(
+  (
+    SELECT COUNT(*) FROM INFORMATION_SCHEMA.COLUMNS
+    WHERE TABLE_SCHEMA = @dbname
+    AND TABLE_NAME = @tablename
+    AND COLUMN_NAME = @columnname
+  ) > 0,
+  'SELECT ''Column lastboottime already exists'';',
+  'ALTER TABLE machines ADD COLUMN lastboottime DATETIME NULL DEFAULT NULL AFTER lastupdated;'
+));
+
+PREPARE alterIfNotExists FROM @preparedStatement;
+EXECUTE alterIfNotExists;
+DEALLOCATE PREPARE alterIfNotExists;
+
+-- Verify the column was added
+SELECT COLUMN_NAME, DATA_TYPE, IS_NULLABLE, COLUMN_DEFAULT
+FROM INFORMATION_SCHEMA.COLUMNS
+WHERE TABLE_SCHEMA = DATABASE()
+AND TABLE_NAME = 'machines'
+AND COLUMN_NAME = 'lastboottime';