Fix startup crash: add DEFAULT to new watched_boost column

ddl-auto=update generated "ALTER TABLE scoring_config ADD COLUMN
watched_boost ... NOT NULL" with no default, which Postgres rejects on the
already-populated singleton config row ("column contains null values").
That aborted the migration, so every later query against scoring_config
failed with "column watched_boost does not exist" - including
/api/recommendations. @ColumnDefault backfills the existing row instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/main/java/com/recommendarr/entity/ScoringConfig.java

@@ -3,6 +3,7 @@ package com.recommendarr.entity;
 import jakarta.persistence.*;
 import lombok.Getter;
 import lombok.Setter;
+import org.hibernate.annotations.ColumnDefault;
 
 /**
  * Singleton settings row (id always = 1) holding the configurable weights
@@ -42,7 +43,12 @@ public class ScoringConfig {
      * (>=90% progress), on top of any rewatch amplification. Being watched
      * is a weight boost only - it never determines source membership.
      */
+    // ColumnDefault ensures the ALTER TABLE ... ADD COLUMN ... NOT NULL that
+    // ddl-auto=update generates on an already-populated table carries a
+    // DEFAULT, so Postgres can backfill the existing row instead of failing
+    // with "column contains null values".
     @Column(nullable = false)
+    @ColumnDefault("0.5")
     private double watchedBoost = 0.5;
 
     /**