docs: [#315] Update backup verification guide with MySQL-specific learnings

- Document expected PROCESS privilege warning in MySQL backups (not fatal)
- Add actual MySQL backup output example with warning message
- Clarify that backup completes successfully despite warning
- Detail MySQL SQL dump verification steps with actual output
- Show how to verify compressed SQL file contains valid database structure
- Add checkmarks confirming expected backup file properties
- Improve clarity on MySQL vs SQLite backup file differences

Author: josecelano

docs/e2e-testing/manual/backup-verification.md

@@ -223,6 +223,17 @@ docker compose run --rm backup
 [2026-02-03 19:05:25] ==========================================
 ```
 
+**For MySQL deployments, you may see this warning** (this is **expected and not fatal**):
+
+```text
+[2026-02-03 19:47:32] Starting MySQL backup: tracker@mysql:3306
+mysqldump: Error: 'Access denied; you need (at least one of) the PROCESS privilege(s) for this operation' when trying to dump tablespaces
+[2026-02-03 19:47:32] MySQL backup completed: /backups/mysql/mysql_20260203_194732.sql.gz
+[2026-02-03 19:47:32]   Size: 4.0K
+```
+
+The warning appears because the backup user (`tracker_user`) has all necessary permissions for table backup, but lacks the PROCESS privilege for tablespace metadata. The backup still completes successfully with all table data intact.
+
 ### Step 7: Verify Backup Files Were Created
 
 ```bash
@@ -265,18 +276,43 @@ ssh -i fixtures/testing_rsa -o StrictHostKeyChecking=no -o UserKnownHostsFile=/d
 
 ### Step 9: Inspect Backup Files (MySQL)
 
-For MySQL deployments, verify the SQL dump was created:
+For MySQL deployments, verify the SQL dump was created with valid content:
 
 ```bash
+# List MySQL backup files
 ssh -i fixtures/testing_rsa -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null \
   torrust@$INSTANCE_IP "ls -lh /opt/torrust/storage/backup/mysql/ | grep '\.sql\.gz'"
 
-# Check file size is reasonable (should be > 0 bytes)
+# Verify SQL structure (decompress and inspect first lines)
 ssh -i fixtures/testing_rsa -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null \
-  torrust@$INSTANCE_IP "du -h /opt/torrust/storage/backup/mysql/*.sql.gz"
+  torrust@$INSTANCE_IP "zcat /opt/torrust/storage/backup/mysql/mysql_*.sql.gz | head -20"
 ```
 
-**Expected**: Compressed SQL dump file (`mysql_<timestamp>.sql.gz`) with reasonable size (> 0 bytes)
+**Expected output**:
+
+File listing shows: `mysql_<timestamp>.sql.gz` with reasonable size (typically 0.5-2 KB for test database)
+
+SQL content preview shows valid MySQL dump headers:
+
+```text
+/*M!999999\- enable the sandbox mode */
+-- MariaDB dump 10.19-11.8.3-MariaDB, for debian-linux-gnu (x86_64)
+--
+-- Host: mysql    Database: tracker
+-- Server version       8.4.8
+
+/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
+/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
+/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
+/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
+```
+
+This confirms:
+
+- ✅ SQL dump is valid and compressed
+- ✅ Contains MySQL 8.4 database structure
+- ✅ Table definitions are included
+- ✅ File is restorable using `mysql < backup.sql`
 
 ### Step 10: Verify Backup Container Logs