feat: ship the AI assistant as a first-party in-process plugin

Moves src/backend/ai and its routes into plugins/ai, mounted through a
small /ai dispatcher instead of its own port. Moves the ai-enabled and
ai-private-endpoints settings routes out of user-settings-routes.ts
into the plugin too, so it has no leftover core dependents.

plugins/ai/backend/gating.ts

@@ -0,0 +1,70 @@
+import type { NextFunction, Response } from "express";
+import type { AuthenticatedRequest } from "../../../src/types/index.js";
+import {
+  createCurrentSettingsRepository,
+  createCurrentUserPreferenceRepository,
+} from "../../../src/backend/database/repositories/factory.js";
+
+/**
+ * Three gates, all checked on the server.
+ *
+ * The admin global is a hard kill switch: when it is off the feature does not
+ * exist for anyone, regardless of what any user has enabled. It defaults to
+ * false so upgrading an existing install turns nothing on by surprise.
+ *
+ * Mirrors the shape of isSharingEnabledForHost in the session-sharing routes,
+ * where the global also wins over the per-entity setting.
+ */
+
+export const AI_GLOBAL_ENABLED_KEY = "ai_globally_enabled";
+
+export async function isAiGloballyEnabled(): Promise<boolean> {
+  return createCurrentSettingsRepository().getBoolean(
+    AI_GLOBAL_ENABLED_KEY,
+    false,
+  );
+}
+
+export interface AiAccess {
+  enabled: boolean;
+  allowReadOnlyCommands: boolean;
+}
+
+export async function resolveAiAccess(userId: string): Promise<AiAccess> {
+  const globalEnabled = await isAiGloballyEnabled();
+  if (!globalEnabled) {
+    return { enabled: false, allowReadOnlyCommands: false };
+  }
+
+  const preferences =
+    await createCurrentUserPreferenceRepository().findByUserId(userId);
+
+  return {
+    // Null means the user was never asked, which is not consent.
+    enabled: preferences?.aiAssistantEnabled === true,
+    allowReadOnlyCommands: preferences?.aiReadOnlyCommands === true,
+  };
+}
+
+/** Rejects any AI request unless both gates are open. */
+export function createAiGate() {
+  return async (
+    req: AuthenticatedRequest,
+    res: Response,
+    next: NextFunction,
+  ): Promise<void> => {
+    if (!req.userId) {
+      res.status(401).json({ error: "Authentication required" });
+      return;
+    }
+
+    const access = await resolveAiAccess(req.userId);
+    if (!access.enabled) {
+      res.status(403).json({ error: "The AI assistant is not enabled" });
+      return;
+    }
+
+    (req as AuthenticatedRequest & { aiAccess?: AiAccess }).aiAccess = access;
+    next();
+  };
+}