feat: redesign metadata as KV store, add custom properties and WOPI Discovery

Replace one-to-one FileMetadata table with flexible ObjectMetadata KV pairs,
add custom property definitions, WOPI Discovery auto-configuration, and
per-extension action URL support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sqlmodels/physical_file.py

@@ -34,6 +34,9 @@ class PhysicalFileBase(SQLModelBase):
     checksum_md5: str | None = Field(default=None, max_length=32)
     """MD5校验和（用于文件去重和完整性校验）"""
 
+    checksum_sha256: str | None = Field(default=None, max_length=64)
+    """SHA256校验和"""
+
 
 class PhysicalFile(PhysicalFileBase, UUIDTableBaseMixin):
     """