feat(security): modernize SSH key algorithm support with Ed25519

Replace deprecated DSA key support with modern SSH key algorithms,
prioritizing Ed25519 as the most secure option.

Changes:
- Add load_ssh_private_key() helper function in common.py
- Support Ed25519 (preferred), ECDSA, and RSA key types
- Remove deprecated and insecure DSA key support
- Update all SSH key loading across backup drivers:
  * common.py: do_preexec, do_postexec, run_remote_command
  * backup_mysql.py
  * backup_pgsql.py
  * backup_sqlserver.py
  * backup_oracle.py
  * backup_samba4.py
- Add ssh_port parameter to preexec/postexec connections
- Update README.md with SSH key generation instructions
- Document supported algorithms and migration path

Algorithm priority:
1. Ed25519 (most secure, modern, fast, timing-attack resistant)
2. ECDSA (secure, widely supported)
3. RSA (legacy support, requires 2048+ bits)

Security improvements:
- Eliminates vulnerable DSA algorithm (1024-bit limit, FIPS deprecated)
- Prioritizes elliptic curve cryptography (Ed25519, ECDSA)
- Provides clear error messages for unsupported key types
- Maintains backward compatibility with existing RSA keys

Documentation:
- Add SSH key generation examples to README.md
- Update expected directory structure to show Ed25519 keys
- Add migration notes in SECURITY_IMPROVEMENTS.md
- Include key generation commands for all supported types

Breaking change:
- DSA keys are no longer supported and will fail with clear error message
- Users must migrate to Ed25519, ECDSA, or RSA (4096-bit recommended)

Migration:
```bash
# Generate new Ed25519 key
ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519

# Copy to remote servers
ssh-copy-id -i ~/.ssh/id_ed25519.pub user@remote
```

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

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

README.md

@@ -29,8 +29,8 @@ Expected structure
       ├── tisbackup-config.ini  <-- backups config
       └── tisbackup_gui.ini     <-- tisbackup config
     └─ssh/
-      ├── id_rsa                <-- SSH Key
-      └── id_rsa.pub            <-- SSH PubKey
+      ├── id_ed25519            <-- SSH Private Key (Ed25519 recommended)
+      └── id_ed25519.pub        <-- SSH Public Key
     compose.yaml
 ```
 
@@ -72,8 +72,30 @@ services:
 
 ## Configuration
 
- * Provide an SSH key and store it in `./ssh`
+### SSH Keys
+
+ * **Generate SSH keys** (Ed25519 recommended):
+   ```bash
+   # Ed25519 (most secure, recommended)
+   ssh-keygen -t ed25519 -f ./ssh/id_ed25519 -C "tisbackup@yourserver"
+
+   # Or ECDSA (also secure)
+   ssh-keygen -t ecdsa -b 521 -f ./ssh/id_ecdsa -C "tisbackup@yourserver"
+
+   # Or RSA (legacy, minimum 2048 bits)
+   ssh-keygen -t rsa -b 4096 -f ./ssh/id_rsa -C "tisbackup@yourserver"
+   ```
+   **⚠️ Note:** DSA keys are no longer supported due to security vulnerabilities
+
+ * Copy public key to remote servers:
+   ```bash
+   ssh-copy-id -i ./ssh/id_ed25519.pub root@remote-server
+   ```
+
+### Configuration Files
+
  * Setup config files in the `./config` directory
+
  * **SECURITY**: Generate and set a secure Flask secret key:
    ```bash
    # Generate a secure random secret key
@@ -99,7 +121,8 @@ server_name=srvads.poudlard.lan
 remote_dir=/var/lib/samba/
 compression=True
 ;exclude_list="/proc/**","/sys/**","/dev/**"
-private_key=/config_ssh/id_rsa
+# Use Ed25519 key (recommended), or ECDSA/RSA (DSA not supported)
+private_key=/config_ssh/id_ed25519
 ssh_port = 22
 ```