feat: manage OS images in database instead of hardcoded values

- Add os_images table with linode_image_id and vultr_os_id columns
- Support Ubuntu (24.04, 22.04), Debian (11-13), AlmaLinux (8-9),
  Rocky Linux (8-9), and Fedora 42
- AlmaLinux and Rocky Linux added as CentOS migration alternatives
- Default OS changed from ubuntu_22_04 to ubuntu_24_04
- Fix Vultr OS IDs (1743=22.04, 2284=24.04)
- Remove hardcoded OS validation, validate against DB
- Return available OS list in error message for invalid image

Migration: migrations/003_os_images.sql

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/handlers/provision.ts

@@ -47,9 +47,13 @@ function validateProvisionRequest(body: unknown): {
     return { valid: false, error: 'label must be 64 characters or less' };
   }
 
-  const validOsImages = ['ubuntu_22_04', 'ubuntu_20_04', 'debian_11', 'debian_12'];
-  if (data.image !== undefined && !validOsImages.includes(data.image as string)) {
-    return { valid: false, error: `image must be one of: ${validOsImages.join(', ')}` };
+  // Basic type validation for image - actual validation against DB done in ProvisioningService
+  if (data.image !== undefined && typeof data.image !== 'string') {
+    return { valid: false, error: 'image must be a string' };
+  }
+
+  if (data.image && (data.image as string).length > 50) {
+    return { valid: false, error: 'image key must be 50 characters or less' };
   }
 
   if (data.dry_run !== undefined && typeof data.dry_run !== 'boolean') {