Share with the directory only when the snapshot changed, or half-hourly (#211)

A scan wakes the sharer, and a scan runs every half minute or so since each checks only the
feeds that are due: Tower sent the same 1,471 feeds every 33 seconds and the directory rewrote
its rows each time. The snapshot, with the address and token it goes to, is compared with what
last went; the same again waits for SHARE_EVERY.

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

CHANGELOG.md

@@ -20,6 +20,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `docker-compose.yml` is a sample to start from, ipx alone on SQLite with its folders beside the file, instead of a copy of the author's server (#208).
 - The source is a Cargo workspace: the daemon in `crates/ipx`, feed fetching and parsing in `crates/feed`. `cargo build --release` still makes `target/release/ipx` (#202).
 
+### Fixed
+
+- ipx sends the directory its feeds when they change, and every half hour, instead of after every scan, which was every half minute or so (#211).
+
 ## [0.12.0] - 2026-10-11
 
 ### Added