Add timezone support to database schema and queries

- Add timezone column (VARCHAR(50) DEFAULT 'UTC') to users table
- Add default_timezone row to system_settings seed data
- Add idx_users_timezone index for user timezone lookups
- Add UpdateUserTimezone and GetSystemTimezone queries
- Regenerate sqlc code (models, querier, queries.sql.go)
- Reuse existing UpdateSystemSetting for system timezone updates
  instead of creating a redundant UpdateSystemTimezone query

database/schema/schema.sql

@@ -32,6 +32,7 @@ CREATE TABLE IF NOT EXISTS users (
     theme VARCHAR(50) DEFAULT 'tokyo-night',
     max_devices INTEGER DEFAULT 10,
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    timezone VARCHAR(50) DEFAULT 'UTC',
     updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
 );
 
@@ -47,7 +48,8 @@ CREATE TABLE IF NOT EXISTS system_settings (
 -- Insert default system settings
 INSERT INTO system_settings (setting_key, setting_value, description) VALUES
 ('scan_poll_interval_seconds', '60', 'How often to scan all libraries in minutes'),
-('auto_scan_enabled', 'true', 'Whether auto-scanning is enabled system-wide')
+('auto_scan_enabled', 'true', 'Whether auto-scanning is enabled system-wide'),
+('default_timezone', 'UTC', 'System default timezone')
 ON CONFLICT (setting_key) DO NOTHING;
 
 -- Create refresh_tokens table
@@ -457,6 +459,7 @@ CREATE TABLE IF NOT EXISTS reading_history (
 -- Create indexes for better query performance
 CREATE INDEX IF NOT EXISTS idx_users_email ON users(email);
 CREATE INDEX IF NOT EXISTS idx_users_username ON users(username);
+CREATE INDEX IF NOT EXISTS idx_users_timezone ON users(timezone);
 CREATE INDEX IF NOT EXISTS idx_library_types_name ON library_types(name);
 CREATE INDEX IF NOT EXISTS idx_refresh_tokens_token ON refresh_tokens(token);
 CREATE INDEX IF NOT EXISTS idx_refresh_tokens_user_id ON refresh_tokens(user_id);