feat(db): add system_settings table and remove per-user scan settings

- Add system_settings table with id, setting_key, setting_value, description, updated_at
- Insert default settings: scan_frequency_minutes=60, auto_scan_enabled=true
- Remove scan_frequency_minutes and auto_scan_enabled from users table
- Migrate from per-user scan settings to system-wide settings

This change enables centralized scan configuration for all libraries
while removing individual user scan preferences.

Database schema changes require container recreation:
  podman compose down -v
  podman compose up -d

database/schema/schema.sql

@@ -26,13 +26,25 @@ CREATE TABLE users (
     last_name VARCHAR(255),
     role VARCHAR(20) NOT NULL DEFAULT 'user' CHECK (role IN ('admin', 'user')),
     theme VARCHAR(50) DEFAULT 'tokyo-night',
-    scan_frequency_minutes INTEGER DEFAULT 60,
-    auto_scan_enabled BOOLEAN DEFAULT true,
     max_devices INTEGER DEFAULT 10,
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
     updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
 );
 
+-- Create system_settings table
+CREATE TABLE system_settings (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    setting_key VARCHAR(100) UNIQUE NOT NULL,
+    setting_value TEXT NOT NULL,
+    description TEXT,
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+
+-- Insert default system settings
+INSERT INTO system_settings (setting_key, setting_value, description) VALUES
+('scan_frequency_minutes', '60', 'How often to scan all libraries in minutes'),
+('auto_scan_enabled', 'true', 'Whether auto-scanning is enabled system-wide');
+
 -- Create refresh_tokens table
 CREATE TABLE refresh_tokens (
     id UUID PRIMARY KEY DEFAULT gen_random_uuid(),