fix(discover): cast nullable params so Postgres can infer type

The discover @Query used the `(:param is null or col >= :param)` idiom.
For the timestamp (publishedAfter) and numeric (minRatio) params, Postgres
threw "could not determine data type of parameter $1" because the bare
`$1 is null` placeholder is untyped — returning HTTP 500 and breaking the
whole /discover page. (The search() query survives because its nullable
params are only bigint/varchar, which Postgres can null-type.) Wrap the
two problematic params in cast(... as timestamp/big_decimal) in the
is-null check so the type is explicit. ORDER BY ... nulls last / fetch
first were never the problem.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main/java/com/hlab/yanalyst/domain/channel/ChannelVideoRepository.java

@@ -55,8 +55,8 @@ public interface ChannelVideoRepository extends JpaRepository<ChannelVideo, Long
      */
     @Query("select v from ChannelVideo v where "
             + "v.interestStatus <> 'EXCLUDED' and "
-            + "(:publishedAfter is null or v.publishedAt >= :publishedAfter) and "
-            + "(:minRatio is null or v.viewsPerSubRatio >= :minRatio) and "
+            + "(cast(:publishedAfter as timestamp) is null or v.publishedAt >= :publishedAfter) and "
+            + "(cast(:minRatio as big_decimal) is null or v.viewsPerSubRatio >= :minRatio) and "
             + "(:source is null or v.source = :source) and "
             + "(:shortsOnly = false or v.isShorts = true) and "
             + "(:unprocessedOnly = false or v.interestStatus in ('NEW','REVIEWING'))")