refactor(storage): remove MinIO backend, filesystem-only storage

Remove the MinIO/S3 storage backend entirely. The filesystem backend is
fully implemented, already used in production, and a migrate-storage tool
exists for any remaining MinIO deployments to migrate beforehand.

Changes:
- Delete MinIO client implementation (internal/storage/storage.go)
- Delete migrate-storage tool (cmd/migrate-storage, scripts/migrate-storage.sh)
- Remove MinIO service, volumes, and env vars from all Docker Compose files
- Simplify StorageConfig: remove Endpoint, AccessKey, SecretKey, Bucket,
  UseSSL, Region fields; add SILO_STORAGE_ROOT_DIR env override
- Change all SQL COALESCE defaults from 'minio' to 'filesystem'
- Add migration 020 to update column defaults to 'filesystem'
- Remove minio-go/v7 dependency (go mod tidy)
- Update all config examples, setup scripts, docs, and tests

scripts/setup-host.sh

@@ -30,7 +30,6 @@ INSTALL_DIR="/opt/silo"
 CONFIG_DIR="/etc/silo"
 GO_VERSION="1.24.0"
 DB_HOST="${SILO_DB_HOST:-psql.example.internal}"
-MINIO_HOST="${SILO_MINIO_HOST:-minio.example.internal}"
 
 log_info() { echo -e "${BLUE}[INFO]${NC} $*"; }
 log_success() { echo -e "${GREEN}[OK]${NC} $*"; }
@@ -165,11 +164,6 @@ if [[ ! -f "${ENV_FILE}" ]]; then
 # Database: silo, User: silo
 SILO_DB_PASSWORD=
 
-# MinIO credentials (${MINIO_HOST})
-# User: silouser
-SILO_MINIO_ACCESS_KEY=silouser
-SILO_MINIO_SECRET_KEY=
-
 # Authentication
 # Session secret (required when auth is enabled)
 SILO_SESSION_SECRET=
@@ -225,10 +219,7 @@ echo ""
 echo "2. Verify database connectivity:"
 echo "   psql -h ${DB_HOST} -U silo -d silo -c 'SELECT 1'"
 echo ""
-echo "3. Verify MinIO connectivity:"
-echo "   curl -I http://${MINIO_HOST}:9000/minio/health/live"
-echo ""
-echo "4. Run the deployment:"
+echo "3. Run the deployment:"
 echo "   sudo ${INSTALL_DIR}/src/scripts/deploy.sh"
 echo ""
 echo "After deployment, manage the service with:"