fix(database): restore api_cache.cached_at read by the playback freshness gate

Main stopped compiling: the v21 column drop treated cached_at as
never-read, but the fresh-cache-first playback metadata gate
(ApiCacheSingleton.getIfFresh, from the playback start-latency fix)
reads it on every Plex and Jellyfin playback start, and a later
cleanup removed the put() stamp the gate depends on.

Reinstate the column, keep the v21 migration to the connections
half only (it never shipped in a release), restore the explicit
put() timestamp - on conflict the upsert only updates the
companion's columns, so without it a refreshed row would keep its
original write time and read as permanently stale - and copy
cached_at through the pinned-metadata rescope statement again. The
v21 migration test now pins that api_cache is left untouched.

lib/services/api_cache.dart

@@ -141,7 +141,15 @@ abstract class ApiCache {
   Future<void> put(ServerId serverId, String endpoint, Map<String, dynamic> data) async {
     final key = _buildKey(serverId, endpoint);
     final encoded = await tryIsolateRun(() => jsonEncode(data));
-    await _db.into(_db.apiCache).insertOnConflictUpdate(ApiCacheCompanion(cacheKey: Value(key), data: Value(encoded)));
+    // The explicit stamp matters: on conflict the upsert only updates the
+    // companion's columns, so relying on the column default would leave a
+    // refreshed row carrying its original write time and [getIfFresh] would
+    // treat just-refetched data as stale.
+    await _db
+        .into(_db.apiCache)
+        .insertOnConflictUpdate(
+          ApiCacheCompanion(cacheKey: Value(key), data: Value(encoded), cachedAt: Value(DateTime.now())),
+        );
   }
 
   Future<void> deleteForServer(ServerId serverId) async {