🐾 feat(settings): register kairos-mail source bucket for ingest

Kairos is about to start announcing rendered mail documents to
POST /library/api/ingest/ with source="kairos-mail". Without a registry
entry, unknown sources fall back to the daedalus bucket and every worker
fetch fails — this adds the KAIROS_S3_* env vars (pattern-copy of the
spelunker block) and the SOURCE_S3_BUCKETS["kairos-mail"] entry, plumbed
through .env.example and the worker service in docker-compose.

Settings-only; no code changes.

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

docker-compose.yaml

@@ -346,6 +346,13 @@ services:
       - SPELUNKER_S3_REGION_NAME=${SPELUNKER_S3_REGION_NAME}
       - SPELUNKER_S3_USE_SSL=${SPELUNKER_S3_USE_SSL}
       - SPELUNKER_S3_VERIFY=${SPELUNKER_S3_VERIFY}
+      - KAIROS_S3_ENDPOINT_URL=${KAIROS_S3_ENDPOINT_URL}
+      - KAIROS_S3_ACCESS_KEY_ID=${KAIROS_S3_ACCESS_KEY_ID}
+      - KAIROS_S3_SECRET_ACCESS_KEY=${KAIROS_S3_SECRET_ACCESS_KEY}
+      - KAIROS_S3_BUCKET_NAME=${KAIROS_S3_BUCKET_NAME}
+      - KAIROS_S3_REGION_NAME=${KAIROS_S3_REGION_NAME}
+      - KAIROS_S3_USE_SSL=${KAIROS_S3_USE_SSL}
+      - KAIROS_S3_VERIFY=${KAIROS_S3_VERIFY}
       # Celery / RabbitMQ
       - CELERY_BROKER_URL=${CELERY_BROKER_URL}
       - CELERY_RESULT_BACKEND=${CELERY_RESULT_BACKEND}