feat(storage): define FileStore interface and refactor to use it

Extract a FileStore interface from the concrete *storage.Storage MinIO
wrapper so the API layer is storage-backend agnostic.

- Define FileStore interface in internal/storage/interface.go
- Add Exists method to MinIO Storage (via StatObject)
- Add compile-time interface satisfaction check
- Change Server.storage and ServerState.storage to FileStore interface
- Update NewServer and NewServerState signatures
- Add Backend and FilesystemConfig fields to StorageConfig
- Add backend selection switch in main.go (minio/filesystem/unknown)
- Update config.example.yaml with backend field

The nil-interface pattern is preserved: when storage is unconfigured,
store remains a true nil FileStore (not a typed nil pointer), so all
existing if s.storage == nil checks continue to work correctly.

Closes #126

config.example.yaml

@@ -17,12 +17,17 @@ 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"
 
 schemas:
   # Directory containing YAML schema files