configs.services.ntfy: add ntfy.lassul.us as UnifiedPush distributor

ntfy.sh's free tier counts every UnifiedPush message against the
subscriber: 250 messages/day shared by all UnifiedPush apps on the phone.
Once used up it answers 429, phonetpm can no longer wake the phone and
every SSH login and deploy fails. Run our own ntfy on neoprism instead,
anonymous like ntfy.sh so app servers publish without accounts.

Rate limits: ntfy 2.29 defaults stay, except for an exemption.
- visitor-message-daily-limit is 0 (unlimited), only the request limiter
applies.
- visitor-subscriber-rate-limiting is off (ntfy.sh turns it on), so
publishes to up* topics are charged to the publisher, not to the phone.
The phone only spends requests on its subscription connections (30
allowed, the app uses one per server).
- The request limit is 60 burst, then one per 5s, per publisher IP.
Synapse on neoprism sends every Matrix push of the phone from one
address; encrypted and bridged rooms notify per message, so bridge
catch-up can exceed that. visitor-request-limit-exempt-hosts exempts
neoprism's own addresses. phonetpm daemons publish one wake per
connection and stay far below the limit.
- behind-proxy: ntfy takes the right-most X-Forwarded-For address, the
one nginx appends, so visitors are not all 127.0.0.1 and a client
cannot spoof its address.

The module enables attachments by default (attachment-cache-dir), set it
empty to keep them off. nginx settings follow ntfy's example config
(buffering off, 3m timeouts against the 45s keepalive, websockets).

No backup: the state dir (/var/lib/private/ntfy-sh) holds the message
cache (12h, disposable) and an auth db without users.