fix(plugins): a plugin that can't start live says why and offers a restart (#6087)

Saving an enabled plugin threw away hot_reload_plugin's (ok, message), so a
plugin that failed to load live read "Enabled, not loaded" with nothing
more. Now:
- a live-load failure is recorded with its reason, as a boot failure is, and
  logged; a later success clears it;
- any save of an enabled plugin that isn't running tries to start it, not
  only the off-to-on toggle;
- enabling one already running is a success, not a failure;
- the plugin's page offers "Restart to load" while it is enabled and not
  running.

Quart never marks the app as having served its first request, so
register_blueprint after startup is allowed: hot-loading is not refused
wholesale, as #6087 suspected.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/core/plugin_manager.py

@@ -305,16 +305,29 @@ def load_plugins(app: "Quart") -> None:
 def hot_reload_plugin(app: "Quart", name: str) -> tuple[bool, str]:
     """Activate a plugin that was just enabled without restarting the app.
 
-    This works for plugins that were NOT previously loaded. For plugins that
-    are already registered (blueprint already mounted), a full restart is
-    required to pick up code changes — use restart_app() in that case.
+    A plugin that can't be loaded live is recorded with its reason, as a
+    failed boot load is, so its settings page says why and offers a restart
+    rather than reading "Enabled, not loaded" with nothing more (#6087).
 
     Returns (success, message).
     """
+    ok, message = _hot_load(app, name)
+    if ok:
+        _FAILED_PLUGINS.pop(name, None)
+    else:
+        _FAILED_PLUGINS[name] = message
+        logger.warning("Plugin %r could not be loaded live: %s", name, message)
+    return ok, message
+
+
+def _hot_load(app: "Quart", name: str) -> tuple[bool, str]:
+    """Load one plugin into the running app. A plugin already loaded (enabled,
+    disabled and enabled again without a restart) is still running, so that
+    is a success; picking up its code changes needs restart_app()."""
     import steward
 
     if name in _LOADED_PLUGINS:
-        return False, "Plugin already loaded — restart required to apply updates"
+        return True, "Plugin already running"
 
     plugin_dirs = [Path(d).resolve() for d in app.config["PLUGIN_DIRS"]]
     plugin_path = resolve_plugin_path(plugin_dirs, name)
@@ -373,9 +386,9 @@ def hot_reload_plugin(app: "Quart", name: str) -> tuple[bool, str]:
             )
             all_dirs = discover_all_in(plugin_dirs)
             run_plugin_migrations(app.config["DATABASE_URL"], all_dirs)
-        except Exception:
+        except Exception as exc:
             logger.exception("Plugin %r: migration failed during hot-reload", name)
-            return False, "Migration failed — check logs"
+            return False, f"Migration failed: {exc}"
 
     # Import (file-path load avoids stdlib name collisions)
     try: