build: add lib-recur to :app, and a dmfs v23 fixture for the import

The own store expands recurrences itself, so lib-recur is a direct
dependency now rather than something :provider drags in. Still pinned at
0.12.2 — 0.16.0 removed RecurrenceSet.

scripts/make_import_fixture.py writes the tasks.db the one-shot import
will be tested against: the provider's DATABASE_VERSION 23 schema, seeded
with the cases the import has to get right (a task with no UID, a deleted
row, a recurring series, an all-day task, a subtask, an alarm property,
and a list under a real CalDAV account). The provider is being deleted, so
a fixture is the only way to keep testing against the schema it wrote.

app/build.gradle.kts

@@ -169,6 +169,12 @@ dependencies {
     implementation(libs.androidx.navigation.compose)
     ksp(libs.hilt.compiler)
 
+    // RFC 5545 recurrence expansion, in-process. Pulled in directly rather than
+    // through :provider: the own store expands occurrences itself. Pinned at
+    // 0.12.2 — 0.16.0 removed RecurrenceSet. rfc5545-datetime comes with it and
+    // is part of its API surface, so it isn't declared separately.
+    implementation(libs.dmfs.lib.recur)
+
     implementation(libs.androidx.room.runtime)
     implementation(libs.androidx.room.ktx)
     ksp(libs.androidx.room.compiler)