feat: [#21] complete database backup automation implementation

## Database Backup Automation - FULLY COMPLETED

### ✅ Core Implementation
- **mysql-backup.sh**: Comprehensive MySQL backup script with compression, retention, logging
- **deploy-app.sh**: Integrated backup automation via setup_backup_automation() in run_stage()
- **Environment templates**: Added ENABLE_DB_BACKUPS and BACKUP_RETENTION_DAYS variables
- **Cron automation**: Automated backup scheduling integrated into deployment workflow

### ✅ Key Features Implemented
- Single-transaction MySQL dumps for data consistency
- Automatic gzip compression to save storage space
- Configurable retention policy (BACKUP_RETENTION_DAYS environment variable)
- Comprehensive error handling and logging for monitoring
- Docker Compose environment integration
- Conditional deployment based on ENABLE_DB_BACKUPS flag

### ✅ Testing & Validation
- Manual end-to-end testing with backup content verification
- Automated scheduling tested with cron job modification
- Log output validation and error handling verification
- Created comprehensive manual testing guide

### ✅ Documentation
- **database-backup-testing-guide.md**: Complete manual testing procedures
- **21-complete-application-installation-automation.md**: Updated status to reflect completion
- Progress updated from 67% to 83% (10/12 components complete)
- Phase 3 marked as fully completed with testing documentation

### 🎯 Production Ready
- Backup automation is fully functional and production-ready
- Zero manual configuration required when ENABLE_DB_BACKUPS=true
- Integrates seamlessly with existing twelve-factor deployment workflow
- All CI tests pass, shellcheck clean, comprehensive error handling

**Next Phase**: SSL certificate automation (manual scripts for admins)

Author: josecelano

application/share/bin/mysql-backup.sh

@@ -0,0 +1,106 @@
+#!/bin/bash
+# MySQL database backup script for Torrust Tracker
+# Creates daily MySQL dumps with automatic cleanup and logging
+
+set -euo pipefail
+
+# Configuration
+APP_DIR="/home/torrust/github/torrust/torrust-tracker-demo/application"
+BACKUP_DIR="/var/lib/torrust/mysql/backups"
+DATE=$(date +%Y%m%d_%H%M%S)
+LOG_PREFIX="[$(date '+%Y-%m-%d %H:%M:%S')]"
+
+# Change to application directory
+cd "$APP_DIR"
+
+# Source environment variables from the deployment location
+ENV_FILE="/var/lib/torrust/compose/.env"
+if [[ -f "$ENV_FILE" ]]; then
+    # shellcheck source=/dev/null
+    source "$ENV_FILE"
+else
+    echo "$LOG_PREFIX ERROR: Environment file not found at $ENV_FILE"
+    exit 1
+fi
+
+# Validate required environment variables
+if [[ -z "${MYSQL_ROOT_PASSWORD:-}" ]]; then
+    echo "$LOG_PREFIX ERROR: MYSQL_ROOT_PASSWORD not set in environment"
+    exit 1
+fi
+
+if [[ -z "${MYSQL_DATABASE:-}" ]]; then
+    echo "$LOG_PREFIX ERROR: MYSQL_DATABASE not set in environment"
+    exit 1
+fi
+
+# Use BACKUP_RETENTION_DAYS from environment, default to 7 days
+RETENTION_DAYS="${BACKUP_RETENTION_DAYS:-7}"
+
+# Validate retention days is numeric
+if ! [[ "$RETENTION_DAYS" =~ ^[0-9]+$ ]]; then
+    echo "$LOG_PREFIX WARNING: BACKUP_RETENTION_DAYS '$RETENTION_DAYS' is not numeric, using default 7 days"
+    RETENTION_DAYS=7
+fi
+
+# Create backup directory if it doesn't exist
+mkdir -p "$BACKUP_DIR"
+
+# Create backup filename
+BACKUP_FILE="torrust_tracker_backup_${DATE}.sql"
+BACKUP_PATH="$BACKUP_DIR/$BACKUP_FILE"
+
+echo "$LOG_PREFIX Starting MySQL backup: $BACKUP_FILE"
+
+# Check if MySQL container is running
+if ! docker compose --env-file "$ENV_FILE" ps mysql | grep -q "Up"; then
+    echo "$LOG_PREFIX ERROR: MySQL container is not running"
+    exit 1
+fi
+
+# Create MySQL dump
+echo "$LOG_PREFIX Creating database dump..."
+if docker compose --env-file "$ENV_FILE" exec -T mysql mysqldump \
+    -u root -p"$MYSQL_ROOT_PASSWORD" \
+    --single-transaction \
+    --routines \
+    --triggers \
+    --add-drop-database \
+    --databases "$MYSQL_DATABASE" > "$BACKUP_PATH"; then
+    echo "$LOG_PREFIX Database dump created successfully"
+else
+    echo "$LOG_PREFIX ERROR: Failed to create database dump"
+    rm -f "$BACKUP_PATH"
+    exit 1
+fi
+
+# Compress the backup
+echo "$LOG_PREFIX Compressing backup..."
+if gzip "$BACKUP_PATH"; then
+    COMPRESSED_BACKUP="${BACKUP_PATH}.gz"
+    echo "$LOG_PREFIX Backup compressed: $(basename "$COMPRESSED_BACKUP")"
+    echo "$LOG_PREFIX Backup size: $(du -h "$COMPRESSED_BACKUP" | cut -f1)"
+else
+    echo "$LOG_PREFIX ERROR: Failed to compress backup"
+    rm -f "$BACKUP_PATH"
+    exit 1
+fi
+
+# Clean up old backups
+echo "$LOG_PREFIX Cleaning up old backups (retention: $RETENTION_DAYS days)..."
+OLD_BACKUPS_COUNT=$(find "$BACKUP_DIR" -name "torrust_tracker_backup_*.sql.gz" -mtime +"$RETENTION_DAYS" | wc -l)
+
+if [[ "$OLD_BACKUPS_COUNT" -gt 0 ]]; then
+    find "$BACKUP_DIR" -name "torrust_tracker_backup_*.sql.gz" -mtime +"$RETENTION_DAYS" -delete
+    echo "$LOG_PREFIX Removed $OLD_BACKUPS_COUNT old backup(s)"
+else
+    echo "$LOG_PREFIX No old backups to remove"
+fi
+
+# Show current backup status
+CURRENT_BACKUPS_COUNT=$(find "$BACKUP_DIR" -name "torrust_tracker_backup_*.sql.gz" | wc -l)
+TOTAL_BACKUP_SIZE=$(du -sh "$BACKUP_DIR" 2>/dev/null | cut -f1 || echo "unknown")
+
+echo "$LOG_PREFIX Backup completed successfully"
+echo "$LOG_PREFIX Current backups: $CURRENT_BACKUPS_COUNT files, total size: $TOTAL_BACKUP_SIZE"
+echo "$LOG_PREFIX Backup location: $COMPRESSED_BACKUP"