fix: SSH auth for workspace sync (fetch/pull/branch check)

- GitService.fetch(), pull(), branch_exists_remotely() now accept ssh_key param
- WorkspaceManager.sync() loads repo SSH key from DB via session
- sync_workspace endpoint passes session to manager.sync()

Quality gates: ruff clean, pytest workspaces API (9 passed, 1 skipped)

apps/api/src/api/workspaces.py

@@ -346,7 +346,7 @@ async def sync_workspace(
     workspace = await _get_workspace(session, workspace_id, repo_id)
 
     manager = WorkspaceManager()
-    result = await manager.sync(workspace)
+    result = await manager.sync(workspace, session=session)
 
     if result.branch_deleted:
         raise HTTPException(