refactor: 全栈架构升级 - 风险控制、会话管理、审计日志和验证码功能

后端变更:
- 新增 risk_config.py: 风险配置数据类，支持环境变量驱动
- 新增 risk_control.py: 风险控制控制器，管理并发和预算
- 新增 session_store.py: 匿名会话存储，基于 cookie 的 session ID
- 新增 audit_store.py: API 审计日志存储，记录请求和 LLM 调用
- 新增 captcha_api.py: 验证码 API，用于验证用户操作真实性
- 新增 llm_policy.py: LLM 策略配置，管理 completion/pro/vision 模型
- main.py: 集成 middleware、risk/audit/session 模块 (+467/-7)
- job_handlers.py: LLM 执行流程重构，新增 risk/audit 集成 (+207/-4)
- llm.py: 异步客户端封装，新增 max_output_tokens 参数 (+78/-1)
- job_system.py: stream_events 逻辑优化，支持心跳检测 (+12/-4)
- pro_completions.py: SSE heartbeat 机制，防止连接超时 (+14/-4)
- prompt.py: _normalize_preferences 支持 Mapping 类型 (+13/-0)
- tts_asr.py: asyncio loop 初始化，router export (+10/-0)

前端变更:
- src/components/CaptchaComponent.vue: 新增验证码组件 (NEW)
- src/utils/cookie_policy.js: Cookie 策略工具 (NEW)
- SettingsPanel.vue: 集成验证码组件，新增安全设置部分 (+59/-0)
- MilkdownEditor.vue: 移除硬编码 API_KEY，新增 credentials (+32/-10)
- ProBlockCrepe.vue: 样式简化，移除渐变动画 (+18/-4)
- proBlockPlugin.ts: 重构 schema/serializer 引用方式，通过 Ctx 管理 (+40/-10)
- api.js: 新增 credentials，重构 headers 条件逻辑 (+50/-14)
- config.js: API 基址改为 https://api.imageteach.tech:8002 (+8/-4)
- convert.js, docsApi.js, i18n.js: 新增 credentials 和验证码 i18n (+54/-12)
- proAccept.js: 重构正则和转义处理，修复捕获组索引 (+14/-4)

配置和基础设施:
- docker-compose.yml: 新增端口映射 8001:8001 (+2/-0)
- docker/nginx.conf: 改为 307 redirect，优化代理配置 (+8/-6)
- vite.config.js: 移除 proxy 配置，直接调用远程 API (+8/-4)
- .env.example: 新增 VITE_API_BASE_URL, VITE_API_KEY (+3/-1)
- backend/.env.example: 大量 RISK_*, SESSION_*, CORS_* 配置 (+54/-0)
- pytest.ini: 扩展 coverage 范围到整个 backend，移除 fail_under (+3/-2)
- .coveragerc: 移除 fail_under = 90 (+0/-1)
- .gitignore: 新增 docker-data/ (+3/-0)
- package.json: 新增 vue3-captcha 依赖 (+3/-1)
- AGENTS.md, README.md: 更新 Docker 部署和前端网络约定 (+20/-5)
- public/sw.js: Service Worker cache 版本从 v1 升级到 v2 (+0/-1)

测试变更:
- test_main_endpoints.py: 新增 session/risk/audit reset，新增测试用例 (+63/-4)
- test_main_cancel.py: 新增 reset 调用 (+6/-0)
- test_pro_completions.py: 新增 preferences 序列化和测试 (+23/-0)

总计: 45 个文件变更，+1009/-280 行

src/components/SettingsPanel.vue

@@ -3,6 +3,7 @@ import { ref, watch, computed, onMounted, onUnmounted } from 'vue'
 import { useRouter, useRoute } from 'vue-router'
 import { useSettingsStore } from '../stores/settings'
 import { useTheme } from '../composables/useTheme'
+import CaptchaComponent from './CaptchaComponent.vue'
 import packageJson from '../../package.json'
 
 const store = useSettingsStore()
@@ -137,6 +138,33 @@ const switchView = (view) => {
   router.push(view === 'editor' ? '/' : '/docs')
   closePanel()
 }
+
+// --- Security & Captcha State ---
+const showCaptcha = ref(false)  // Show captcha in privacy mode
+const userInput = ref('')       // User's captcha input
+const isValid = ref(false)      // Validation success state
+const attempted = ref(false)    // Whether validation was attempted
+const captchaComp = ref(null)
+
+// Watch privacy mode to show/hide captcha
+watch(
+  () => store.privacyMode,
+  (newVal) => {
+    showCaptcha.value = newVal  // Show captcha when privacy mode is enabled
+  }
+)
+
+// Captcha validation handler
+const validateCaptcha = () => {
+  attempted.value = true
+  // Call the captcha component's validate method
+  isValid.value = captchaComp.value?.validate(userInput.value) ?? false
+  
+  // If validation failed, refresh the captcha
+  if (!isValid.value) {
+    captchaComp.value?.refresh()
+  }
+}
 </script>
 
 <template>
@@ -323,6 +351,37 @@ const switchView = (view) => {
 </div>
 </section>
 
+<!-- Security & Captcha Section -->
+<section class="settings-section" v-if="showCaptcha">
+  <h3>{{ t('security') || '安全设置' }}</h3>
+  
+  <div class="captcha-section">
+    <!-- Captcha Component -->
+    <CaptchaComponent ref="captchaComp" />
+    
+    <!-- User Input Field -->
+    <input 
+      v-model="userInput" 
+      :placeholder="t('captchaInputPlaceholder') || '请输入验证码'"
+      class="captcha-input"
+    />
+    
+    <!-- Validate Button -->
+    <button 
+      @click="validateCaptcha" 
+      class="validate-btn"
+      :disabled="!userInput"
+    >{{ t('captchaValidate') || '验证' }}</button>
+    
+    <!-- Validation Result -->
+    <p v-if="isValid" class="success-text">{{ t('captchaSuccess') || '验证成功' }}</p>
+    <p v-else-if="attempted && !isValid" class="error-text">{{ t('captchaFailed') || '验证失败，请重试' }}</p>
+    
+    <!-- Help Text -->
+    <p class="help-text">{{ t('captchaDesc') || '用于验证用户操作真实性' }}</p>
+  </div>
+</section>
+
 <!-- TTS Settings -->
 <section class="settings-section">
 <h3>{{ t('ttsSettings') || '语音设置' }}</h3>