Setup wizard P2: per-plugin config schema + settings-first creds

- Plugin contract gains get_config_schema(); the plugins list API returns it.
  Employees plugin declares its directory-DB fields (host/name/user + password).
- employee_connection reads host/name/user settings-first (env fallback); the
  password stays env-only.
- Setup wizard Features step renders each enabled plugin's config: non-secret
  fields save to settings; secrets are never stored - the wizard emits .env
  lines to paste. Fixed the plugins-list data path (data.plugins).
- Settings PUT now upserts (creates the row on first write) so plugin-config
  keys can be saved without pre-seeding.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

shopdb/plugins/__init__.py

@@ -143,6 +143,10 @@ class PluginManager:
                     meta = temp.meta
                     state = self.registry.get(name)
 
+                    try:
+                        config_schema = temp.get_config_schema()
+                    except Exception:
+                        config_schema = []
                     available.append({
                         'name': meta.name,
                         'version': meta.version,
@@ -151,7 +155,8 @@ class PluginManager:
                         'dependencies': meta.dependencies,
                         'installed': state is not None,
                         'enabled': state.enabled if state else False,
-                        'installedat': state.installed_at if state else None
+                        'installedat': state.installed_at if state else None,
+                        'config_schema': config_schema,
                     })
                 except Exception as e:
                     logger.warning(f"Error inspecting plugin {name}: {e}")