Security: Add path traversal protection to plugin operations

Add defense-in-depth validation to prevent path traversal attacks when
enabling/installing plugins via the UI or CLI.

Layer 1 - Input Validation (enable_plugin):
- Validate plugin names with regex ^[a-zA-Z0-9_-]+$
- Reject reserved names (., .., empty string)
- Blocks attempts like ../../../etc/passwd at entry point

Layer 2 - Path Boundary Check (load_manifest):
- Verify resolved manifest path stays within plugins_dir
- Defense in depth even if name validation is bypassed
- Compatible with Python 3.8+ (fallback for < 3.9)

Risk Assessment: LOW
- Requires admin role (already trusted)
- Multiple existing validation layers
- No known exploitation in the wild

Rationale: Defense in depth best practice. Admins are trusted, but input
validation should not rely solely on authorization checks.

Tested:
- Valid names pass (backups, my-plugin, plugin-name)
- Path traversal blocked (../../../etc/passwd)
- Reserved names blocked (..)
- Both validation layers confirmed working

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

shopdb/plugins/loader.py

@@ -84,6 +84,29 @@ class PluginLoader:
             return self._manifests[name]
 
         manifest_path = self.plugins_dir / name / 'manifest.json'
+
+        # Security: Ensure resolved path stays within plugins_dir boundary
+        # Defense in depth against path traversal, even if name was validated
+        resolved_manifest = manifest_path.resolve()
+        resolved_plugins = self.plugins_dir.resolve()
+
+        # Python 3.9+ has is_relative_to, fallback for older versions
+        try:
+            if not resolved_manifest.is_relative_to(resolved_plugins):
+                raise PluginNotFoundError(
+                    f'Plugin path escapes plugins directory: {name}',
+                    plugin_name=name,
+                )
+        except AttributeError:
+            # Python < 3.9 fallback using relative_to
+            try:
+                resolved_manifest.relative_to(resolved_plugins)
+            except ValueError:
+                raise PluginNotFoundError(
+                    f'Plugin path escapes plugins directory: {name}',
+                    plugin_name=name,
+                )
+
         if not manifest_path.exists():
             raise PluginNotFoundError(
                 f'Plugin {name} has no manifest.json at {manifest_path}',