feat(hot-rank): add per-channel hot ranking functionality

Add channel-specific hot post filtering and ranking across the posts
listing flow. Introduce refreshChannelRanks to compute per-channel hot
scores, new Redis cache keys for channel hot ranks, and update
repository/service/handler layers to accept channelID filtering
parameters. Update HotRankWorker dependency injection to include
ChannelRepository.

internal/cache/keys.go

@@ -57,6 +57,14 @@ func HotRankTopIDsKey() string {
 	return HotRankTopIDs
 }
 
+func HotRankZSetChannelKey(channelID string) string {
+	return fmt.Sprintf("%s:ch:%s", HotRankZSet, channelID)
+}
+
+func HotRankTopIDsChannelKey(channelID string) string {
+	return fmt.Sprintf("%s:ch:%s", HotRankTopIDs, channelID)
+}
+
 func PostListKey(postType string, userID string, page, pageSize int) string {
 	if userID == "" {
 		return fmt.Sprintf("%s:%s:%d:%d", PrefixPostList, postType, page, pageSize)