feat: Initial commit for OnEver Drive centralized backup system with Windows PyQt6 agent and Proxmox LXC deployment

backend/app/storage/base.py

@@ -0,0 +1,64 @@
+from abc import ABC, abstractmethod
+from typing import List, Tuple, Dict, Any, Optional
+
+class BaseStorageProvider(ABC):
+    """Abstract interface for OnEver Drive storage backends (Local FS, S3, MinIO, etc.)."""
+
+    @abstractmethod
+    async def init_session_storage(self, session_code: str) -> None:
+        """Prepares temporary storage directory for an incoming upload session."""
+        pass
+
+    @abstractmethod
+    async def save_chunk(
+        self,
+        session_code: str,
+        chunk_index: int,
+        chunk_data: bytes,
+        expected_sha256: Optional[str] = None
+    ) -> bool:
+        """Saves a single chunk, validates its hash, and returns True on success."""
+        pass
+
+    @abstractmethod
+    async def get_received_chunks(self, session_code: str) -> List[int]:
+        """Returns the list of indices of all successfully stored chunks for a session."""
+        pass
+
+    @abstractmethod
+    async def assemble_file(
+        self,
+        session_code: str,
+        client_code: str,
+        job_code: str,
+        filename: str,
+        total_chunks: int,
+        expected_sha256: str
+    ) -> Tuple[str, str, int]:
+        """
+        Assembles all stored chunks in order into the final destination file,
+        calculates full streaming SHA-256 hash, and verifies integrity.
+        Returns: (relative_storage_path, actual_sha256, file_size_bytes).
+        Raises ValueError if integrity check fails or chunks are missing.
+        """
+        pass
+
+    @abstractmethod
+    async def delete_session_temp(self, session_code: str) -> None:
+        """Cleans up temporary chunks after assembly or cancellation."""
+        pass
+
+    @abstractmethod
+    async def delete_backup_file(self, relative_path: str) -> bool:
+        """Deletes a backup file from storage."""
+        pass
+
+    @abstractmethod
+    async def get_file_path(self, relative_path: str) -> str:
+        """Resolves absolute path for reading/restoring."""
+        pass
+
+    @abstractmethod
+    async def get_storage_stats(self) -> Dict[str, Any]:
+        """Returns storage capacity stats: {total_bytes, used_bytes, free_bytes, usage_percent}."""
+        pass