feat: update config samples with SQLite and Viper support

- Add SQLite configuration options to config.json.sample
- Create config.mysql.json.sample for MySQL deployments
- Add security comments for sensitive configuration
- Include environment variable examples and documentation
- Add Viper configuration comments and usage examples

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

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

config.mysql.json.sample

@@ -0,0 +1,19 @@
+{
+  "WebUrl": "https://domain.com",
+  "Address": "",
+  "Port": 8080,
+  "ApiPrefix": "",
+  "KeyFile": "",
+  "CertFile": "",
+  "_comment_database": "MySQL configuration for production use",
+  "DatabaseDriver": "mysql",
+  "DatabaseAddress": "localhost:3306",
+  "Database": "openaccounting",
+  "User": "openaccounting",
+  "Password": "openaccounting",
+  "_comment_mailgun": "Mailgun configuration for email sending",
+  "MailgunDomain": "mg.domain.com",
+  "MailgunKey": "",
+  "MailgunEmail": "noreply@domain.com",
+  "MailgunSender": "Sender"
+}