feat(ai): clinic-wide AI kill-switch (admin-controlled)

- new org_ai_policy table + GET/PUT /api/ai/policy (read for any member,
  write owner/admin only); migration 0018
- /api/chat hard-blocks (403) when AI is off for the caller
- Settings → AI "Availability" section: enable AI, or disable for
  employees only (owners/admins keep access); read-only for non-admins
- sidebar, command palette and route guard hide/redirect the AI chat
  when it's disabled for the current user

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/routes/chat.ts

@@ -21,6 +21,7 @@ import {
 import { recordActivity } from "../services/activity.js";
 import * as aiChat from "../services/ai-chat.js";
 import { getAiSettings } from "../services/ai/config.js";
+import { aiAllowedFor, getPolicy } from "../services/ai/policy.js";
 import { resolveModel } from "../services/ai/provider.js";
 import { createChatTools } from "../services/ai/tools.js";
 import { createVeil } from "../services/ai/veil.js";
@@ -146,6 +147,13 @@ chatRouter.post("/", async (req, res, next) => {
       return;
     }
 
+    // Honour the clinic's AI kill-switch — employees can't reach the agent even
+    // by bypassing the (also-gated) UI.
+    const policy = await getPolicy(req.organizationId!);
+    if (!aiAllowedFor(policy, req.memberRole)) {
+      throw new HttpError(403, "The AI assistant is disabled for your account.");
+    }
+
     const settings = await getAiSettings(req.user!.id);
     const modelId = requestedModel || settings.defaultModel;
     const resolved = resolveModel(settings, modelId);