fix: rollback Enabled on activation failure + fatal on_init + rollback test

1. SetPluginEnabled(true): after DeactivatePlugin, also call Disable(name)
   to rollback in-memory Enabled state (not just config).
2. on_init failure is now fatal for ActivatePlugin — returns error
   and rolls back scheduler + VM (was incorrectly non-fatal).
3. TestSetPluginEnabled_BrokenPlugin_Rollback: end-to-end test with
   broken plugin (invalid interval), verifies error + not Active +
   not Enabled + not in config.

internal/core/plugins/manager_lifecycle.go

@@ -12,6 +12,7 @@ import (
 // ActivatePlugin fully activates a plugin: creates Lua VM, loads main.lua, starts scheduler.
 // Only works if plugin is installed and enabled but not yet active.
 // Returns error if VM creation, script loading, scheduler setup, or on_init hook fails.
+// on_init failure is fatal — the plugin cannot run without proper initialization.
 func (m *Manager) ActivatePlugin(name string) error {
 	for i := range m.plugins {
 		p := &m.plugins[i]
@@ -56,8 +57,12 @@ func (m *Manager) ActivatePlugin(name string) error {
 
 		if hookName, ok := p.Meta.Hooks["on_init"]; ok {
 			if err := vm.CallHook(hookName); err != nil {
-				// on_init failure is non-fatal for activation — log but continue
-				log.Printf("[plugins] %s: on_init error: %v", name, err)
+				// on_init failure is fatal — rollback activation
+				p.scheduler.Stop()
+				p.scheduler = nil
+				p.vm.Close()
+				p.vm = nil
+				return fmt.Errorf("on_init for %q: %w", name, err)
 			}
 		}