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Build me web push for my own application, and treat token hygiene as the main job.

STACK
- Node with Fastify, SQLite via better-sqlite3
- The web-push library for VAPID; do not implement the encryption yourself

SUBSCRIPTION
- Service worker registers and posts the subscription to the server
- Ask for permission only after a user action that implies interest — never on page load, and the README says why
- Store endpoint, keys, user id, user agent bucket and created time

SENDING
- A queue with a worker; batch by endpoint host so one slow push service does not block the rest
- Per-notification: title, body, icon, url, and a tag so a repeat replaces rather than stacks
- TTL set deliberately; a notification that is late is often worse than one that never arrives

TOKEN HYGIENE, WHICH IS THE ACTUAL WORK
- A 404 or 410 from the push service means the subscription is dead: delete it immediately
- A 429 means back off for that host, not for everything
- A subscription that has failed five times in a row is retired and recorded as retired
- Report the live subscription count weekly; a list that only grows is a list that is lying

SEGMENTS
- Simple attribute filters evaluated at send time against your own user table

SCHEDULING
- Deliver in the recipient's local morning by storing their timezone, not by guessing from an IP

Include a README on VAPID keys, the retirement rules and the delivery rates you actually measured.