refactor(api): unify error handling and split upload/oauth god modules

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.

internal/apps/upload/exports.go

@@ -0,0 +1,86 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package upload
+
+import (
+	"github.com/Rain-kl/Wavelet/internal/apps/upload/cache"
+	"github.com/Rain-kl/Wavelet/internal/apps/upload/filesrv"
+	"github.com/Rain-kl/Wavelet/internal/apps/upload/handler"
+	uploadstats "github.com/Rain-kl/Wavelet/internal/apps/upload/stats"
+	uploadtask "github.com/Rain-kl/Wavelet/internal/apps/upload/task"
+	"github.com/Rain-kl/Wavelet/internal/apps/upload/util"
+	"github.com/Rain-kl/Wavelet/internal/task"
+)
+
+// HTTP handlers
+var (
+	UploadFile             = handler.UploadFile
+	DownloadFile           = handler.DownloadFile
+	BatchDownloadFiles     = handler.BatchDownloadFiles
+	ListFiles              = handler.ListFiles
+	DeleteFile             = handler.DeleteFile
+	GetDistinctUploadTypes = handler.GetDistinctUploadTypes
+	ListMyFiles            = handler.ListMyFiles
+	DeleteMyFile           = handler.DeleteMyFile
+	UpdateMyFile           = handler.UpdateMyFile
+	GetFileStats           = handler.GetFileStats
+	ServeFileByID          = filesrv.ServeFileByID
+)
+
+// Cache management
+var (
+	ResetAccessCaches              = cache.ResetAccessCaches
+	PublishAccessCacheInvalidation = cache.PublishAccessCacheInvalidation
+)
+
+// Stats
+var (
+	ApplyUploadStatsAdd    = uploadstats.ApplyUploadStatsAdd
+	ApplyUploadStatsRemove = uploadstats.ApplyUploadStatsRemove
+	RebuildUploadStats     = uploadstats.RebuildUploadStats
+)
+
+// Utilities
+var (
+	CompressImageToWebP = util.CompressImageToWebP
+	ValidateS3Key       = util.ValidateS3Key
+)
+
+// Task identifiers and metadata
+const (
+	StorageMigrationTask = uploadtask.StorageMigrationTask
+	SystemCleanupTask    = uploadtask.SystemCleanupTask
+	WarmImageCacheTask   = uploadtask.WarmImageCacheTask
+)
+
+var (
+	// StorageMigrationMeta describes the storage migration async task.
+	StorageMigrationMeta = uploadtask.StorageMigrationMeta
+	// SystemCleanupMeta describes the orphaned upload cleanup task.
+	SystemCleanupMeta = uploadtask.SystemCleanupMeta
+	// WarmImageCacheMeta describes the image compression cache warmup task.
+	WarmImageCacheMeta = uploadtask.WarmImageCacheMeta
+)
+
+// MigrationHandler executes storage migration tasks.
+type MigrationHandler = uploadtask.MigrationHandler
+
+// SystemCleanupHandler removes orphaned upload files.
+type SystemCleanupHandler = uploadtask.SystemCleanupHandler
+
+// WarmImageCacheHandler pre-warms compressed image caches.
+type WarmImageCacheHandler = uploadtask.WarmImageCacheHandler
+
+// WarmImageCachePayload is the payload for image cache warmup tasks.
+type WarmImageCachePayload = uploadtask.WarmImageCachePayload
+
+// Ensure task handler types implement required interfaces.
+var (
+	_ task.TaskHandler = (*MigrationHandler)(nil)
+	_ task.TaskHandler = (*SystemCleanupHandler)(nil)
+	_ interface {
+		task.TaskHandler
+		ValidatePayload([]byte) ([]byte, error)
+	} = (*WarmImageCacheHandler)(nil)
+)