Push notifications: period soon/late reminders, test button

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -26,6 +26,25 @@ boundary.
 
 The logic lives in `static/cycle.js` and is tested in `tests/test_cycle.js`.
 
+## Notifications
+
+Web Push, sent by `notify.js` (Node, no dependencies: built-in crypto and
+SQLite). It reuses `static/cycle.js`, so reminders always match the app.
+
+- `flowtrack-notify daily` runs once a day. It sends "period expected soon"
+  once per cycle, `remindDays` (1-7, default 2) before the predicted start,
+  and "period is late" once per cycle after the expected day passes.
+- The owner turns notifications on per device in Settings, where there is
+  also a test button. On iPhone this only works from the Home Screen app
+  (iOS 16.4 or later).
+- The VAPID key pair is created on first start and stored in the database.
+  Replacing it invalidates every subscription.
+- The server only accepts subscriptions on Apple, Google, Mozilla and
+  Microsoft push hosts, because it POSTs to whatever endpoint it stores.
+
+Enable it with `--push-contact https://your.site` (the NixOS module does
+this through `push.contact`).
+
 ## Access
 
 Nginx must overwrite `Remote-User` with the Authelia username. Users listed
@@ -49,6 +68,8 @@ services.flowtrack = {
   editors = [ "kayla" ];
   viewers = [ "fred" ];
   backup.directory = "/mnt/disk2/backups/flowtrack";  # nightly, keeps 30
+  push.contact = "https://flowtrack.example.org";
+  push.time = "*-*-* 09:00:00 Europe/London";
 };
 ```