feat(backend): unified response helpers, refresh token, security hardening

- new resp package (OK/Page/Fail/BadRequest/.../InternalErr): unified error
  envelope {detail, code?}, sanitized 500s (full error logged server-side,
  generic message to client - stops DB error leakage), pagination envelope
  {items,total,page,page_size}; migrate 200+ inline c.JSON error sites
- refresh token: 7d rotation tokens (SHA-256 hashed at rest), public
  POST /auth/refresh with single-use rotation, POST /auth/logout revokes
  all user refresh tokens (endpoint previously missing - frontend 404)
- security: gh-ost command line password masked in task status payload;
  Instance.MarshalJSON guarantees encrypted passwords never leave the API
  (removes reliance on per-handler manual masking); production (DEBUG=false)
  with default JWT secret now auto-generates and persists a random secret
- safe.Go recover wrapper for background goroutines (instance probe,
  gh-ost async)
- delete dead service layer (zero callers)
- fix pre-existing go vet unkeyed bson.E literals

backend/internal/safe/safe.go

@@ -0,0 +1,20 @@
+// Package safe 提供带 recover 的后台 goroutine 包装：
+// 裸 go func() 一旦 panic 会击穿整个进程，后台任务必须经由 safe.Go 启动。
+package safe
+
+import (
+	"log"
+	"runtime/debug"
+)
+
+// Go 启动带 recover 的后台任务；panic 只记录堆栈，不影响进程。
+func Go(name string, fn func()) {
+	go func() {
+		defer func() {
+			if r := recover(); r != nil {
+				log.Printf("[safe:%s] panic recovered: %v\n%s", name, r, debug.Stack())
+			}
+		}()
+		fn()
+	}()
+}