fix: score basé sur l'observation la plus proche du spot, pas la dernière globale

impact_score stocke source_observation_id — la lecture trie par distance(obs→spot)
ASC pour garantir que Barbade n'écrase jamais Guadeloupe. Suppression du garde
"closest wins" au write, remplacé par un ORDER BY déterministe au read.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/Version20260408000000.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20260408000000 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add source_observation_id to impact_score for nearest-observation scoring';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE impact_score ADD COLUMN source_observation_id UUID NULL');
+        $this->addSql('ALTER TABLE impact_score ADD CONSTRAINT fk_impact_score_observation
+            FOREIGN KEY (source_observation_id) REFERENCES sargassum_observation(id) ON DELETE SET NULL');
+        $this->addSql('CREATE INDEX idx_impact_score_source_obs ON impact_score (source_observation_id)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP INDEX idx_impact_score_source_obs');
+        $this->addSql('ALTER TABLE impact_score DROP CONSTRAINT fk_impact_score_observation');
+        $this->addSql('ALTER TABLE impact_score DROP COLUMN source_observation_id');
+    }
+}