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/handler/stub_handlers.go

@@ -5,6 +5,7 @@ import (
 	"net/http"
 	"strings"
 
+	"sql-platform/internal/handler/resp"
 	"sql-platform/internal/model"
 
 	"github.com/gin-gonic/gin"
@@ -20,7 +21,7 @@ func AuditTicketSQL(c *gin.Context) {
 		Database   string `json:"database"`
 	}
 	if err := c.ShouldBindJSON(&input); err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
+		resp.BadRequest(c, err.Error())
 		return
 	}
 
@@ -184,7 +185,7 @@ func SQLFormat(c *gin.Context) {
 		SQL string `json:"sql" binding:"required"`
 	}
 	if err := c.ShouldBindJSON(&input); err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
+		resp.BadRequest(c, err.Error())
 		return
 	}
 
@@ -200,7 +201,7 @@ func SQLAnalyze(c *gin.Context) {
 		DBType string `json:"db_type"`
 	}
 	if err := c.ShouldBindJSON(&input); err != nil {
-		c.JSON(http.StatusBadRequest, gin.H{"detail": err.Error()})
+		resp.BadRequest(c, err.Error())
 		return
 	}