Add notification channels and a provider abstraction

Users get a notification_channels table (replacing users.ntfy_topic); pages fan
out to every enabled channel through a notify.Provider registry, with ntfy as
the first provider. Fallbacks are typed {type, address}. Schema edited in place:
existing databases must be recreated.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

internal/web/static/js/onboarding.js

@@ -36,10 +36,10 @@ export async function load() {
     ]);
     steps = [
       {
-        id: 'topic',
+        id: 'channel',
         title: 'Set where your pages go',
-        text: 'An ntfy topic on your account. Without one, incidents assigned to you page the team’s fallback topic instead of your phone.',
-        done: Boolean(state.me?.user?.ntfy_topic),
+        text: 'A notification channel on your account. Without one, incidents assigned to you page the team’s fallback instead of you.',
+        done: Boolean(firstChannel()),
         action: { label: 'Account', go: '/more' },
       },
       {
@@ -115,10 +115,10 @@ function stepRow(step) {
           class: 'btn-sm', type: 'button', text: step.action.label,
           onclick: () => navigate(step.action.go),
         }),
-        // The topic step is the only one this page can finish by itself, and
+        // The channel step is the only one this page can finish by itself, and
         // the only proof that matters is a phone buzzing.
-        step.id === 'topic' && state.me?.user?.ntfy_topic && h('button', {
-          class: 'btn-sm', type: 'button', text: 'Send a test push',
+        step.id === 'channel' && firstChannel() && h('button', {
+          class: 'btn-sm', type: 'button', text: 'Send a test',
           disabled: busy,
           onclick: sendTest,
         }),
@@ -126,11 +126,15 @@ function stepRow(step) {
   );
 }
 
+// The first channel that is switched on: the one a page would reach first.
+const firstChannel = () => (state.me?.user?.channels || []).find((c) => c.enabled);
+
 async function sendTest() {
   busy = true;
   try {
-    await api.testNotification();
-    testResult = { ok: true, text: 'Sent. If nothing arrives, the topic is wrong or ntfy is not reachable.' };
+    const channel = firstChannel();
+    await api.testChannel(state.me.user.id, channel.id);
+    testResult = { ok: true, text: 'Sent. If nothing arrives, the address is wrong or the server cannot reach the provider.' };
   } catch (err) {
     testResult = { ok: false, text: err.message };
   } finally {