feat(alerts): 数据库连接告警覆盖 MySQL/PG/Redis 主流报错；规则表单三字段对齐

=== 1. 「数据库连接失败」匹配模式 3 → 25 个 ===
原来只匹配 connection refused / reset by peer / too many connections，
漏掉了 PG 的 FATAL: too many clients、Redis 的 NOAUTH / WRONGPASS /
connection pool timeout 等实际最常见的报错。

按数据库分类补齐：
  通用网络层
    connection refused / connection reset by peer / broken pipe /
    no route to host / cannot assign requested address /
    connection timed out / i/o timeout / unexpected EOF /
    server closed the connection unexpectedly
  MySQL
    too many connections / communications link failure /
    driver: bad connection / could not create connection /
    Access denied for user
  PostgreSQL
    FATAL: password authentication failed / sorry, too many clients /
    terminating connection
  Redis
    NOAUTH / WRONGPASS / READONLY you can't write against /
    CLUSTERDOWN / max number of clients reached
  连接池（Java/Go 应用层最先炸的地方）
    connection pool / connection is not available

**刻意不加**的（会匹配到几乎所有网络日志，等于没告警）：
   裸 "EOF"、"timeout"、"error"、"refused"、"deadlock"
阈值保持 1000/15min（模式变多不代表命中变多，都是真错误才命中）。

迁移 v9→v10 只替换"仍是旧默认查询"的规则，用户自定义过的不动。

=== 2. 规则表单三个字段没对齐 ===
现象：统计窗口(分) / 触发阈值(条) / 冷却(分) 三格并排，
但只有中间的「触发阈值」下方多了一行提示文字，
导致中间那格更高、提示文字悬在底部，看起来像没对齐。
修法：三格统一为「标签 + 输入框 + 一行提示（min-h-4 固定高度）」，
底边自然齐平；提示文案改为：
    统计窗口 → 统计最近多少分钟
    触发阈值 → 超过该条数才告警
    冷却    → 冷却内不重复发送
（顺带把阈值提示从"0 = 出现即告警"改掉：默认阈值已无 0，0 反而是危险配置）
小屏改单列堆叠（grid-cols-1 sm:grid-cols-3）。

=== 实测（936a127b1d15）===
  v10 迁移：告警规则 "数据库连接失败" 扩展匹配模式
  线上规则：25 个模式，含 NOAUTH / too many clients，阈值 1000，冷却 240m
  Pod = latest = v0.1 = 936a127b1d15

backend/seed/alert_rules.go

@@ -82,7 +82,9 @@ var builtinRules = []builtinRule{
 	{name: "应用错误激增(error > 10万)", query: `{namespace=~".+"} |~ "(?i)\berror\b"`, windowMin: 15, threshold: 100000, cooldown: 240},
 	{name: "ingress-nginx 5xx 错误", query: `{app=~"ingress-nginx|nginx-ingress-controller"} |~ " (50[0-4]|5[2-9][0-9]) "`, windowMin: 15, threshold: 2000, cooldown: 240},
 	{name: "ingress-nginx 4xx 激增", query: `{app=~"ingress-nginx|nginx-ingress-controller"} |~ " 4[0-9][0-9] "`, windowMin: 15, threshold: 20000, cooldown: 240},
-	{name: "数据库连接失败", query: `{namespace=~".+"} |~ "(?i)(connection refused|connection reset by peer|too many connections)"`, windowMin: 15, threshold: 1000, cooldown: 240},
+	// 数据库连接类错误：覆盖 MySQL / PostgreSQL / Redis 的主流报错文案
+	// （详见函数上方注释，不要加裸 "EOF" / "timeout"，那会匹配到几乎所有网络日志）
+	{name: "数据库连接失败", query: `{namespace=~".+"} |~ "(?i)(connection refused|connection reset by peer|broken pipe|no route to host|cannot assign requested address|connection timed out|i/o timeout|too many connections|too many clients already|max number of clients reached|connection pool|connection is not available|could not create connection|communications link failure|server closed the connection unexpectedly|unexpected EOF|driver: bad connection|FATAL:\\s*(password authentication failed|sorry, too many clients|terminating connection)|NOAUTH|WRONGPASS|READONLY you can.t write|CLUSTERDOWN)"`, windowMin: 15, threshold: 1000, cooldown: 240},
 }
 
 // seedBuiltinAlertRules 为每个平台管理的 LogSource(managed=true)补齐内置规则: