Fix webhook failing to update plan for pre-existing Supabase users

supabase_admin_invite re-sends a magic link for users who already have
an account (e.g. signed up via attestation before paying) but does not
touch their user_metadata — leaving plan as "free" after purchase.

Fix: add supabase_admin_update_user (PUT /auth/v1/admin/users/{id})
and call it after every invite so user_metadata is always stamped with
the correct plan, name, and stripe_customer_id. Idempotent for new and
returning users.

Also fix waitlist_upsert to use on_conflict=email,plan so the upsert
works for users who already have a waitlist row from attestation,
rather than silently failing on duplicate key.

dist/web_stubs.c

@@ -237,6 +237,55 @@ el_val_t supabase_admin_invite(el_val_t project_url, el_val_t service_key, el_va
     return http_post_with_headers(EL_STR(url), body_json, headers);
 }
 
+/*
+ * supabase_admin_update_user — PUT {project_url}/auth/v1/admin/users/{user_id}
+ * with the service-role key to overwrite a user's user_metadata (and any other
+ * top-level fields in body_json).  Unlike /auth/v1/invite, this always writes
+ * the supplied data even when the user already exists.
+ *
+ * body_json example:
+ *   {"user_metadata":{"plan":"founding","stripe_customer_id":"cus_xxx","name":"..."}}
+ *
+ * Returns the raw JSON response from Supabase (includes the updated user object).
+ * Returns "" on transport error.
+ *
+ * Used by the Stripe webhook after supabase_admin_invite to guarantee the
+ * plan is stamped correctly regardless of whether the account was created
+ * before or after payment.
+ */
+el_val_t supabase_admin_update_user(el_val_t project_url, el_val_t service_key,
+                                    el_val_t user_id, el_val_t body_json) {
+    CURL *c = curl_easy_init();
+    if (!c) return EL_STR("");
+    char url[1024];
+    snprintf(url, sizeof(url), "%s/auth/v1/admin/users/%s",
+             EL_CSTR(project_url), EL_CSTR(user_id));
+    char auth_hdr[2048];
+    snprintf(auth_hdr, sizeof(auth_hdr), "Authorization: Bearer %s", EL_CSTR(service_key));
+    char api_hdr[2048];
+    snprintf(api_hdr,  sizeof(api_hdr),  "apikey: %s", EL_CSTR(service_key));
+    struct curl_slist *hdrs = NULL;
+    hdrs = curl_slist_append(hdrs, auth_hdr);
+    hdrs = curl_slist_append(hdrs, api_hdr);
+    hdrs = curl_slist_append(hdrs, "Content-Type: application/json");
+    hdrs = curl_slist_append(hdrs, "Accept: application/json");
+    _stub_resp_t r = {0};
+    curl_easy_setopt(c, CURLOPT_URL, url);
+    curl_easy_setopt(c, CURLOPT_CUSTOMREQUEST, "PUT");
+    curl_easy_setopt(c, CURLOPT_POSTFIELDS, EL_CSTR(body_json));
+    curl_easy_setopt(c, CURLOPT_HTTPHEADER, hdrs);
+    curl_easy_setopt(c, CURLOPT_FOLLOWLOCATION, 1L);
+    curl_easy_setopt(c, CURLOPT_TIMEOUT, 60L);
+    curl_easy_setopt(c, CURLOPT_WRITEFUNCTION, _stub_write);
+    curl_easy_setopt(c, CURLOPT_WRITEDATA, &r);
+    CURLcode rc = curl_easy_perform(c);
+    curl_easy_cleanup(c);
+    curl_slist_free_all(hdrs);
+    if (rc != CURLE_OK) { free(r.buf); return EL_STR(""); }
+    if (!r.buf) return EL_STR("");
+    return EL_STR(r.buf);
+}
+
 /*
  * gcs_get_token — fetch an OAuth2 bearer token.
  *