cleanup: drop dead storage_objects.parent_object_id column + idx_storage_parent

parent_object_id was never written or read by any application code (verified
via repo-wide grep: only the baseline migration and the ORM model referenced
it). The orphan index idx_storage_parent likewise served nothing.

Tree structure is maintained entirely via the materialized path in
storage_objects.relative_path (LIKE-prefix queries in
backend/src/backend/scripts.py: list_workspace_tree, list_workspace_directories).
The column mislead a prior review into proposing an adjacency-list table —
removing it eliminates that temptation for the next reader.

Migration: migrations/versions/f7a8b9c0d1e2_drop_storage_parent.py
- Drops idx_storage_parent first, then parent_object_id (correct order on MySQL).
- MySQL 8.0 has no IF EXISTS on DROP INDEX / DROP COLUMN, so calls are unconditional.
- Updates the storage_objects TABLE COMMENT so the materialized-path warning
  reaches the DB, not just the ORM (Codex review finding).
- Downgrade restores both.

Model: common/src/common/db/models/storage.py
- Removes the dead Index entry and the dead column.
- Refreshes the table comment to flag the materialized-path contract.

Verified:
- alembic upgrade head: applied, head = f7a8b9c0d1e2
- SHOW INDEX / SHOW COLUMNS: 0 rows
- TABLE COMMENT updated in information_schema
- pytest backend/tests: 43 passed

common/src/common/db/models/storage.py

@@ -20,7 +20,6 @@ class StorageObjects(Base):
         Index("fk_storage_created_by", "created_by"),
         Index("idx_storage_content_hash", "content_hash"),
         Index("idx_storage_owner", "owner_user_id", "object_status"),
-        Index("idx_storage_parent", "parent_object_id"),
         Index(
             "idx_storage_workspace_path",
             "workspace_id",
@@ -40,7 +39,7 @@ class StorageObjects(Base):
             "object_key_hash_active",
             unique=True,
         ),
-        {"comment": "Workspace 文件和 RustFS 对象的统一元数据"},
+        {"comment": "Workspace 文件和 RustFS 对象的统一元数据;目录树走 materialized path (relative_path),不要 join 邻接表列——已删除。"},
     )
 
     storage_object_id: Mapped[str] = mapped_column(CHAR(26), primary_key=True)
@@ -86,7 +85,6 @@ class StorageObjects(Base):
         server_default=text("CURRENT_TIMESTAMP(3) ON UPDATE CURRENT_TIMESTAMP(3)"),
     )
     owner_user_id: Mapped[str | None] = mapped_column(CHAR(26))
-    parent_object_id: Mapped[str | None] = mapped_column(CHAR(26))
     relative_path: Mapped[str | None] = mapped_column(
         String(1024), comment="Workspace 相对路径"
     )