TASK-020: Fix log_target security vulnerability (defense in depth)

**Issue:**
- log_target accepted arbitrary paths, allowing authenticated users to write
  files as root via fail2ban (e.g., /etc/cron.d/bangui-pwned)
- fail2ban runs as root and opens files specified in log_target

**Solution:**
1. **Model layer validation:** Already existed in GlobalConfigUpdate, prevents
   invalid paths before reaching service
2. **Service layer validation:** Added defensive check in update_global_config()
   that validates log_target even if model validation is bypassed
3. **New validation helper:** Added validate_log_target() utility that accepts
   special values (STDOUT, STDERR, SYSLOG) or paths within allowed directories

**Changes:**
- app/utils/path_utils.py: Added validate_log_target() helper
- app/services/config_service.py: Added service-layer validation before
  sending command to fail2ban
- backend/tests: Fixed session_secret length issues in fixtures (min 32 chars)
- backend/tests: Added tests for valid special log targets
- Docs/Backend-Development.md: Documented log_target security requirements

**Test Coverage:**
- Model validation rejects /etc/passwd (existing test)
- Model validation accepts STDOUT, STDERR, SYSLOG special values
- Model validation accepts paths in allowed directories
- Service layer validation tested with special values

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/utils/path_utils.py

@@ -38,3 +38,25 @@ def validate_log_path(log_path: str) -> str:
     raise ValueError(
         f"Log path {log_path!r} is outside allowed directories: {allowed_dirs_str}"
     )
+
+
+def validate_log_target(log_target: str) -> str:
+    """Validate that a log target is either a special value or a valid file path.
+
+    Accepts special values (STDOUT, STDERR, SYSLOG) and file paths that resolve
+    to one of the configured allowed log directories.
+
+    Args:
+        log_target: The log target to validate.
+
+    Returns:
+        The validated log target (unchanged).
+
+    Raises:
+        ValueError: If the target is not a special value and not in allowed directories.
+    """
+    if log_target.upper() in ("STDOUT", "STDERR", "SYSLOG"):
+        return log_target
+
+    return validate_log_path(log_target)
+