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

config.example.yaml

@@ -17,17 +17,9 @@ database:
   max_connections: 10
 
 storage:
-  backend: "minio" # "minio" (default) or "filesystem"
-  # MinIO/S3 settings (used when backend: "minio")
-  endpoint: "localhost:9000" # Use "minio:9000" for Docker Compose
-  access_key: "" # Use SILO_MINIO_ACCESS_KEY env var
-  secret_key: "" # Use SILO_MINIO_SECRET_KEY env var
-  bucket: "silo-files"
-  use_ssl: true # Use false for Docker Compose (internal network)
-  region: "us-east-1"
-  # Filesystem settings (used when backend: "filesystem")
-  # filesystem:
-  #   root_dir: "/var/lib/silo/objects"
+  backend: "filesystem"
+  filesystem:
+    root_dir: "/opt/silo/data" # Override with SILO_STORAGE_ROOT_DIR env var
 
 schemas:
   # Directory containing YAML schema files