Reserved feeds

For feeds that only the operator posts to but everyone may subscribe to, such as news or announcements. List the names in NOTEFEED_RESERVED_FEEDS and set NOTEFEED_RESERVED_PASSWORD:

NOTEFEED_RESERVED_FEEDS: news,announcements,updates
NOTEFEED_RESERVED_PASSWORD: a-long-operator-password

At start-up (not at the first request) each listed name that doesn't exist yet is created as a protected feed with that password. Post to it like any protected feed, with X-Feed-Password: <the password> (or Authorization: Bearer first, if the instance has a password too). Its read id is its name, with no obfuscation: /r/news and /r/news/feed.xml are the links to hand out, and they never change, even if the feed is deleted and made again at the next start. A reserved feed deleted while running comes back at the next start.

Nobody can create a listed name by posting: that answers 400 (reserved_feed). Without NOTEFEED_RESERVED_PASSWORD the names are still blocked, but the feeds don't exist. A feed that already had such a name keeps working as it was.

If a reserved name already exists as an open feed, it is not converted: delete its folder in DATA_DIR and restart, so notefeed recreates it protected, and the start-up log warns about it until then.