fix(lint): 修复所有 Go 命名规范问题

- 全局替换 ApiKey → APIKey（类型、字段、方法、变量）
- 修复所有 initialism 命名（API, SMTP, HTML, URL 等）
- 添加所有缺失的包注释
- 修复导出符号的注释格式

主要修改：
- ApiKey → APIKey（所有出现的地方）
- ApiKeyID → APIKeyID
- ApiKeyIDs → APIKeyIDs
- TestSmtpConnection → TestSMTPConnection
- HtmlURL → HTMLURL
- 添加 20+ 个包注释
- 修复 10+ 个导出符号注释格式

验证结果：
- ✓ golangci-lint: 0 issues
- ✓ 单元测试: 通过
- ✓ 集成测试: 通过

backend/internal/pkg/antigravity/client.go

@@ -1,3 +1,4 @@
+// Package antigravity provides a client for the Antigravity API.
 package antigravity
 
 import (

backend/internal/pkg/claude/constants.go

@@ -1,3 +1,4 @@
+// Package claude provides constants and helpers for Claude API integration.
 package claude
 
 // Claude Code 客户端相关常量
@@ -16,13 +17,13 @@ const DefaultBetaHeader = BetaClaudeCode + "," + BetaOAuth + "," + BetaInterleav
 // HaikuBetaHeader Haiku 模型使用的 anthropic-beta header（不需要 claude-code beta）
 const HaikuBetaHeader = BetaOAuth + "," + BetaInterleavedThinking
 
-// ApiKeyBetaHeader API-key 账号建议使用的 anthropic-beta header（不包含 oauth）
-const ApiKeyBetaHeader = BetaClaudeCode + "," + BetaInterleavedThinking + "," + BetaFineGrainedToolStreaming
+// APIKeyBetaHeader API-key 账号建议使用的 anthropic-beta header（不包含 oauth）
+const APIKeyBetaHeader = BetaClaudeCode + "," + BetaInterleavedThinking + "," + BetaFineGrainedToolStreaming
 
-// ApiKeyHaikuBetaHeader Haiku 模型在 API-key 账号下使用的 anthropic-beta header（不包含 oauth / claude-code）
-const ApiKeyHaikuBetaHeader = BetaInterleavedThinking
+// APIKeyHaikuBetaHeader Haiku 模型在 API-key 账号下使用的 anthropic-beta header（不包含 oauth / claude-code）
+const APIKeyHaikuBetaHeader = BetaInterleavedThinking
 
-// Claude Code 客户端默认请求头
+// DefaultHeaders 是 Claude Code 客户端默认请求头。
 var DefaultHeaders = map[string]string{
 	"User-Agent":                                "claude-cli/2.0.62 (external, cli)",
 	"X-Stainless-Lang":                          "js",

backend/internal/pkg/errors/types.go

@@ -1,3 +1,4 @@
+// Package errors provides application error types and helpers.
 // nolint:mnd
 package errors
 

backend/internal/pkg/gemini/models.go

@@ -1,7 +1,6 @@
-package gemini
-
-// This package provides minimal fallback model metadata for Gemini native endpoints.
+// Package gemini provides minimal fallback model metadata for Gemini native endpoints.
 // It is used when upstream model listing is unavailable (e.g. OAuth token missing AI Studio scopes).
+package gemini
 
 type Model struct {
 	Name                       string   `json:"name"`

backend/internal/pkg/geminicli/constants.go

@@ -1,3 +1,4 @@
+// Package geminicli provides helpers for interacting with Gemini CLI tools.
 package geminicli
 
 import "time"

backend/internal/pkg/googleapi/status.go

@@ -1,3 +1,4 @@
+// Package googleapi provides helpers for Google-style API responses.
 package googleapi
 
 import "net/http"

backend/internal/pkg/oauth/oauth.go

@@ -1,3 +1,4 @@
+// Package oauth provides helpers for OAuth flows used by this service.
 package oauth
 
 import (

backend/internal/pkg/openai/constants.go

@@ -1,3 +1,4 @@
+// Package openai provides helpers and types for OpenAI API integration.
 package openai
 
 import _ "embed"

backend/internal/pkg/openai/oauth.go

@@ -327,7 +327,7 @@ func ParseIDToken(idToken string) (*IDTokenClaims, error) {
 	return &claims, nil
 }
 
-// ExtractUserInfo extracts user information from ID Token claims
+// UserInfo represents user information extracted from ID Token claims.
 type UserInfo struct {
 	Email            string
 	ChatGPTAccountID string

backend/internal/pkg/pagination/pagination.go

@@ -1,3 +1,4 @@
+// Package pagination provides types and helpers for paginated responses.
 package pagination
 
 // PaginationParams 分页参数

backend/internal/pkg/response/response.go

@@ -1,3 +1,4 @@
+// Package response provides standardized HTTP response helpers.
 package response
 
 import (

backend/internal/pkg/sysutil/restart.go

@@ -1,3 +1,4 @@
+// Package sysutil provides system-level utilities for process management.
 package sysutil
 
 import (

backend/internal/pkg/usagestats/usage_log_types.go

@@ -1,3 +1,4 @@
+// Package usagestats provides types for usage statistics and reporting.
 package usagestats
 
 import "time"
@@ -10,8 +11,8 @@ type DashboardStats struct {
 	ActiveUsers   int64 `json:"active_users"`    // 今日有请求的用户数
 
 	// API Key 统计
-	TotalApiKeys  int64 `json:"total_api_keys"`
-	ActiveApiKeys int64 `json:"active_api_keys"` // 状态为 active 的 API Key 数
+	TotalAPIKeys  int64 `json:"total_api_keys"`
+	ActiveAPIKeys int64 `json:"active_api_keys"` // 状态为 active 的 API Key 数
 
 	// 账户统计
 	TotalAccounts     int64 `json:"total_accounts"`
@@ -82,10 +83,10 @@ type UserUsageTrendPoint struct {
 	ActualCost float64 `json:"actual_cost"` // 实际扣除
 }
 
-// ApiKeyUsageTrendPoint represents API key usage trend data point
-type ApiKeyUsageTrendPoint struct {
+// APIKeyUsageTrendPoint represents API key usage trend data point
+type APIKeyUsageTrendPoint struct {
 	Date     string `json:"date"`
-	ApiKeyID int64  `json:"api_key_id"`
+	APIKeyID int64  `json:"api_key_id"`
 	KeyName  string `json:"key_name"`
 	Requests int64  `json:"requests"`
 	Tokens   int64  `json:"tokens"`
@@ -94,8 +95,8 @@ type ApiKeyUsageTrendPoint struct {
 // UserDashboardStats 用户仪表盘统计
 type UserDashboardStats struct {
 	// API Key 统计
-	TotalApiKeys  int64 `json:"total_api_keys"`
-	ActiveApiKeys int64 `json:"active_api_keys"`
+	TotalAPIKeys  int64 `json:"total_api_keys"`
+	ActiveAPIKeys int64 `json:"active_api_keys"`
 
 	// 累计 Token 使用统计
 	TotalRequests            int64   `json:"total_requests"`
@@ -128,7 +129,7 @@ type UserDashboardStats struct {
 // UsageLogFilters represents filters for usage log queries
 type UsageLogFilters struct {
 	UserID      int64
-	ApiKeyID    int64
+	APIKeyID    int64
 	AccountID   int64
 	GroupID     int64
 	Model       string
@@ -157,9 +158,9 @@ type BatchUserUsageStats struct {
 	TotalActualCost float64 `json:"total_actual_cost"`
 }
 
-// BatchApiKeyUsageStats represents usage stats for a single API key
-type BatchApiKeyUsageStats struct {
-	ApiKeyID        int64   `json:"api_key_id"`
+// BatchAPIKeyUsageStats represents usage stats for a single API key
+type BatchAPIKeyUsageStats struct {
+	APIKeyID        int64   `json:"api_key_id"`
 	TodayActualCost float64 `json:"today_actual_cost"`
 	TotalActualCost float64 `json:"total_actual_cost"`
 }