Phase 1 : pipeline d'ingestion Sentinel-2

- SentinelHubClient : auth OAuth2, Catalog API (cloud coverage),
  Process API avec evalscript AFAI binaire (UINT8, cloud-side)
- IngestionService : orchestration complète — job tracking,
  rejet nuages, téléchargement GeoTIFF, polygonize GDAL,
  simplification PostGIS, persistance SargassumObservation
- IngestSentinelCommand : 5 zones Antilles configurées,
  options --date et --zone, appelable via cron docker exec
- Dockerfile : ajout gdal-bin + python3-gdal
- Correction entity : paramètre immutable retiré de #[ORM\Column]

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

backend/composer.json

@@ -17,11 +17,14 @@
         "phpdocumentor/reflection-docblock": "^6.0",
         "phpstan/phpdoc-parser": "^2.3",
         "symfony/asset": "7.4.*",
+        "symfony/cache": "7.4.*",
         "symfony/console": "7.4.*",
         "symfony/dotenv": "7.4.*",
         "symfony/expression-language": "7.4.*",
         "symfony/flex": "^2",
         "symfony/framework-bundle": "7.4.*",
+        "symfony/http-client": "7.4.*",
+        "symfony/process": "7.4.*",
         "symfony/property-access": "7.4.*",
         "symfony/property-info": "7.4.*",
         "symfony/runtime": "7.4.*",