Enable admin user removals and invitation UX polish

convex/users.ts

@@ -90,6 +90,17 @@ export const listAgents = query({
   },
 });
 
+export const findByEmail = query({
+  args: { tenantId: v.string(), email: v.string() },
+  handler: async (ctx, { tenantId, email }) => {
+    const record = await ctx.db
+      .query("users")
+      .withIndex("by_tenant_email", (q) => q.eq("tenantId", tenantId).eq("email", email))
+      .first();
+    return record ?? null;
+  },
+});
+
 export const deleteUser = mutation({
   args: { userId: v.id("users"), actorId: v.id("users") },
   handler: async (ctx, { userId, actorId }) => {