fix(README): repair empty NIP-9a links, add Node & single-instance constraints

- Replace empty NIP-9a URL (first paragraph and API section header)
  with a link to https://github.com/nostr-protocol/nips/blob/master/9a.md
- Add Node >= 22 requirement to Development section
- Add single-instance constraint note about in-process cron jobs

README.md

@@ -1,6 +1,6 @@
 # Mailship
 
-A Nostr email notification server. Receives events pushed from relays via [NIP-9a](), stores them, and sends daily or weekly digest emails.
+A Nostr email notification server. Receives events pushed from relays via [NIP-9a](https://github.com/nostr-protocol/nips/blob/master/9a.md), stores them, and sends daily or weekly digest emails.
 
 This repo is a fork of [anchor](https://github.com/coracle-social/anchor). It has been built primarily to support email notifications in [Flotilla](https://flotilla.social), but supports alternate branding and could be set up to work with any Nostr relay supporting NIP-9a.
 
@@ -79,7 +79,7 @@ Response: { ok: true }
 ```
 
 ### POST /notify/:id
-NIP-9a relay push callback. Called by relays or NPB when matching events are found.
+[NIP-9a](https://github.com/nostr-protocol/nips/blob/master/9a.md) relay push callback. Called by relays or NPB when matching events are found.
 
 ```
 Body: { id, relay, event? }
@@ -112,6 +112,11 @@ Unsubscribe via link from digest email.
 
 ## Development
 
+**Requirements:** Node >= 22
+
+> **Single instance:** Mailship uses in-process cron jobs for digest scheduling.
+> Running more than one instance concurrently may cause duplicate or missed emails.
+
 ```sh
 pnpm install
 pnpm run build