fix(security): 修复安全审计的 P1/P2/P3 项，含全局复查新发现的两处越权与外跳

P1：
- 外链图片：markdown 渲染层只放行站内与自有 CDN 图片。配置里的
  image_bucket_cdn_host 常写成 host/path，统一归一化后再比对，避免把自家图一起剥掉
- 密码：页面/提问密码最小 6 位；密码比对挪到验证码之后；页面密码失败锁定 5 次/10 分钟
- 客户端 IP：CF-Connecting-IP / EO-Client-IP 只在 TCP 对端可信（回环或 trusted_proxies）
  时采信，公网对端带来的权威头一律忽略

P2/P3：
- 跳转参数收敛到 shared.SafeRedirectTarget：拒反斜杠与控制字符（url.Parse 会把 %0d%0a
  解成 CRLF，伪造 Location），最后再用 path.Clean 验一次前缀
- 注册不再暴露「邮箱已注册 / 已停用」：与全新注册共用同一个成功出口，只在日志留痕
- CSP：默认 enforce，白名单由配置派生，覆盖 umami（运行时注入）、Turnstile 与
  按 sentry_dsn 条件放行的 Sentry CDN
- /user 与 /_ 下发 no-store；补 X-XSS-Protection、Permissions-Policy，并把安全头前移到
  静态资源中间件之前（原先静态文件拿不到 nosniff/HSTS）
- 封禁事实落库 ban_records；429 统一带 Retry-After 与本地化文案
- token 比较改常量时间；探索页页码上限；JSON 响应显式 Content-Type

全局复查补的两处：
- 私聊 SendMessage 的 POST 缺 conv.OwnerID 校验，被拉黑者可换 domain 继续发消息
- 页面密码 next 与登录 to 同源的反斜杠外跳

验收：verify_p1_fixes 4/4、verify_round2 19/19、verify_page_next 6/6、
verify_chat_owner 3/3（含变异对照）；单测与 go build ./... 全通过。

internal/i18n/dict_en.go

@@ -16,21 +16,22 @@ var enDict = map[string]string{
 	"nav.lang":     "Language",
 
 	// Common
-	"common.login":      "Log in",
-	"common.forgot":     "Forgot password",
-	"common.submit":     "Submit",
-	"common.cancel":     "Cancel",
-	"common.save":       "Save",
-	"common.close":      "Close",
-	"common.delete":     "Delete",
-	"common.moveUp":     "Move up",
-	"common.moveDown":   "Move down",
-	"common.share":      "Share",
-	"common.verifying":  "Verifying...",
-	"common.submitting": "Submitting...",
-	"common.uploading":  "Uploading...",
-	"common.lang":       "Language",
-	"common.menu":       "Menu",
+	"common.login":           "Log in",
+	"common.forgot":          "Forgot password",
+	"common.submit":          "Submit",
+	"common.cancel":          "Cancel",
+	"common.save":            "Save",
+	"common.tooManyRequests": "Too many requests. Please try again later.",
+	"common.close":           "Close",
+	"common.delete":          "Delete",
+	"common.moveUp":          "Move up",
+	"common.moveDown":        "Move down",
+	"common.share":           "Share",
+	"common.verifying":       "Verifying...",
+	"common.submitting":      "Submitting...",
+	"common.uploading":       "Uploading...",
+	"common.lang":            "Language",
+	"common.menu":            "Menu",
 
 	"common.pleaseWait": "Please wait...",
 	"common.backHome":   "Back to Home",
@@ -182,9 +183,9 @@ var enDict = map[string]string{
 	"admin.tagInputHint":      "Type and press Enter or comma to add",
 	"admin.tagInputDuplicate": "Already exists, ignored",
 	"admin.pagePassword":      "Page access password",
-	"admin.pagePasswordHint":  "Set a page password (leave empty to disable)",
+	"admin.pagePasswordHint":  "Set a page password (6–30 characters, leave empty to disable)",
 	"admin.askPassword":       "Ask password",
-	"admin.askPasswordHint":   "Set a password for asking (leave empty to disable)",
+	"admin.askPasswordHint":   "Set a password for asking (6–30 characters, leave empty to disable)",
 	"admin.saveProfile":       "Save changes",
 	"admin.updated":           "Profile updated successfully",
 	"admin.logout":            "Log out",
@@ -619,6 +620,7 @@ var enDict = map[string]string{
 	"vote.page.submit":                 "Submit vote",
 	"vote.page.alreadyVoted":           "You have already participated in this poll. Thanks!",
 	"vote.page.resultsTitle":           "Poll Results",
+	"vote.page.anonymousNotice":        "Anonymous poll — one vote per person, results are indicative only.",
 	"vote.page.totalVotes":             "Total",
 	"vote.page.avgRating":              "Average",
 	"vote.page.avgRank":                "Avg rank",
@@ -757,6 +759,7 @@ var enDict = map[string]string{
 
 	// Question box page: visitor-visible (P0)
 	"question.errWrongPassword":     "Wrong password",
+	"question.errPasswordLocked":    "Too many wrong attempts. Please try again in %d minutes.",
 	"question.errAskPasswordWrong":  "Wrong ask password",
 	"question.errLoginRequired":     "The owner only allows registered users to ask questions. Please log in first.",
 	"question.errSelfChat":          "You cannot start a chat with yourself",
@@ -926,6 +929,7 @@ var enDict = map[string]string{
 	"form.profile.name":                 "Nickname is required and must be at most 20 characters",
 	"form.profile.newPassword":          "New password must be at most 30 characters",
 	"form.profile.passwordMax":          "Password must be at most 30 characters",
+	"form.profile.passwordLen":          "Please set a password of 6–30 characters",
 	"form.question.content":             "Question content is required",
 	"form.question.contentTooLong":      "Question content must be within %d characters",
 	"form.question.receiveReplyEmail":   "Please enter a valid email address to receive replies",