store: add list_items, the fourth append-only shape (V-453)

A list is a standing set of short strings under a tag. Not a task, because
milk is not work and the prioritiser must not count it as an errand; not a
fact, because it claims nothing. Nothing predicates over it, so two people
adding to the same list at once costs nothing.

Migration #19, plus AddListItem, ListItems, SetListItemStatus and ClearList.
The live-only unique index is the tasks one, per list: молоко twice before
the shop is one row, молоко again after it was crossed off is a new one.

internal/store/migrations.go

@@ -219,6 +219,27 @@ ALTER TABLE reminders ADD COLUMN next_fire_ts INTEGER;`, // #2
 	// event, and the old rows would otherwise be recited as extra meetings.
 	// The filter is exact — it keeps any key whose summary part still has a
 	// letter or a digit in it.
+	`CREATE TABLE IF NOT EXISTS list_items (
+		id          INTEGER PRIMARY KEY AUTOINCREMENT,
+		created_ts  INTEGER NOT NULL,
+		list        TEXT    NOT NULL,
+		item        TEXT    NOT NULL,
+		norm        TEXT    NOT NULL,
+		source      TEXT    NOT NULL,
+		status      TEXT    NOT NULL DEFAULT 'open' CHECK (status IN ('open','done','dropped')),
+		resolved_ts INTEGER
+	);
+	CREATE UNIQUE INDEX IF NOT EXISTS idx_list_items_live ON list_items (list, norm) WHERE status = 'open';
+	CREATE INDEX IF NOT EXISTS idx_list_items_list ON list_items (list, status, created_ts);`,
+	// #19 — standing lists (Vikunja #453). The fourth append-only shape, after
+	// facts, notes and tasks, and the reason it is its own table rather than a
+	// tag on tasks: milk on the shopping list is not work. Nothing prioritises
+	// it, nothing nudges about it, and the prioritiser must not start counting
+	// groceries as outstanding errands.
+	//
+	// The live-only unique index is the tasks one, per list: saying "молоко"
+	// twice before the shop keeps one row, saying it again next week after the
+	// last one was crossed off writes a new one.
 }
 
 // migrate applies every migration with a number greater than the DB's current