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>

libtisbackup/backup_pgsql.py

@@ -53,11 +53,7 @@ class backup_pgsql(backup_generic):
         else:
             raise Exception("backup destination directory already exists : %s" % self.dest_dir)
 
-        try:
-            mykey = paramiko.RSAKey.from_private_key_file(self.private_key)
-        except paramiko.SSHException:
-            # mykey = paramiko.DSSKey.from_private_key_file(self.private_key)
-            mykey = paramiko.Ed25519Key.from_private_key_file(self.private_key)
+        mykey = load_ssh_private_key(self.private_key)
 
         self.logger.debug(
             '[%s] Trying to connect to "%s" with username root and key "%s"', self.backup_name, self.server_name, self.private_key