feat(api): add hp_regen_since column for persistent HP regeneration

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

apps/api/src/characters/entities/character.entity.ts

@@ -35,6 +35,12 @@ export class Character {
   @Column({ name: 'current_hp', type: 'integer' })
   currentHp!: number;
 
+  // `current_hp` is only exact while this is null (regeneration paused, e.g.
+  // mid-combat). Otherwise it's the HP as of this timestamp -- read it
+  // through CharacterVitalsService.effectiveHp(), never directly.
+  @Column({ name: 'hp_regen_since', type: 'timestamptz', nullable: true })
+  hpRegenSince!: Date | null;
+
   @Column({ name: 'current_location_id', type: 'uuid' })
   currentLocationId!: string;