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

Makefile

@@ -1,8 +1,7 @@
 .PHONY: build run test test-integration clean migrate fmt lint \
         docker-build docker-up docker-down docker-logs docker-ps \
         docker-clean docker-rebuild \
-        web-install web-dev web-build \
-        migrate-storage
+        web-install web-dev web-build
 
 # =============================================================================
 # Local Development
@@ -57,13 +56,6 @@ tidy:
 migrate:
 	./scripts/init-db.sh
 
-# Build and run MinIO → filesystem migration tool
-# Usage: make migrate-storage DEST=/opt/silo/data [ARGS="--dry-run --verbose"]
-migrate-storage:
-	go build -o migrate-storage ./cmd/migrate-storage
-	@echo "Built ./migrate-storage"
-	@echo "Run: ./migrate-storage -config <config.yaml> -dest <dir> [-dry-run] [-verbose]"
-
 # Connect to database (requires psql)
 db-shell:
 	PGPASSWORD=$${SILO_DB_PASSWORD:-silodev} psql -h $${SILO_DB_HOST:-localhost} -U $${SILO_DB_USER:-silo} -d $${SILO_DB_NAME:-silo}
@@ -76,7 +68,7 @@ db-shell:
 docker-build:
 	docker build -t silo:latest -f build/package/Dockerfile .
 
-# Start the full stack (postgres + minio + silo)
+# Start the full stack (postgres + silo)
 docker-up:
 	docker compose -f deployments/docker-compose.yaml up -d
 
@@ -103,9 +95,6 @@ docker-logs-silo:
 docker-logs-postgres:
 	docker compose -f deployments/docker-compose.yaml logs -f postgres
 
-docker-logs-minio:
-	docker compose -f deployments/docker-compose.yaml logs -f minio
-
 # Show running containers
 docker-ps:
 	docker compose -f deployments/docker-compose.yaml ps
@@ -175,7 +164,7 @@ help:
 	@echo ""
 	@echo "Docker:"
 	@echo "  docker-build   - Build Docker image"
-	@echo "  docker-up      - Start full stack (postgres + minio + silo)"
+	@echo "  docker-up      - Start full stack (postgres + silo)"
 	@echo "  docker-down    - Stop the stack"
 	@echo "  docker-clean   - Stop and remove volumes (deletes data)"
 	@echo "  docker-logs    - View all logs"