Resolve user package conflicts by keeping main's service-layer logics and
applying Abort* error handling in routers. Align oauth callback with
listener.EmitAdminLoggedIn from main.
Replace c.JSON(200, response.Err) and middleware gin.H bypasses with
response.Abort* helpers so errors flow through Gin Error chain and
ErrorHandlerMiddleware for OTel trace correlation.
Split oauth/sources.go into domain-focused files and decompose upload
into handler/filesrv/stats/task/cache/storage/util subpackages with a
root facade preserving existing import paths.