Block lists: words that hide items and keep them from downloading (#47)

Each person has a list for every feed they read and one per feed. An item whose title or text
holds one of the words, matched as whole words so "ai" does not hide everything that "said"
anything, is hidden from them and, since the scanner now keeps each subscriber's filters
separate, is fetched only if someone else still wants it.

Whole-word matching is not something LIKE can do on both SQLite and Postgres, so the matches are
worked out in Rust into a `hidden` table whenever a list changes, someone subscribes, or a scan
brings in new items, and the queries only look that table up. Both new tables are tables rather
than columns because create_missing adds tables but never columns. Hidden counts as read for
the reaper and for "others still want this file", since whoever it is hidden from is as done
with it.

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

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Words that hide items. Settings has a list for every feed you read, and each feed's settings a
+  list of its own; an item with one of those words or phrases in its title or text is hidden
+  from you and is not downloaded on your account. Other people's lists never hide anything from
+  you.
+
 ### Fixed
 
 - Swiping to the next or previous item takes a longer swipe, a quarter of the screen, so a