feat: per-key spend accounting and per-task complexity pinning

API keys page: the table now answers "which department spends what" — per-key
requests, tokens in/out and cost for a selectable window (24h/7d/30d) plus a
cumulative all-time cost column, joined from stats by the key's account.

The stats log stays bounded while the accounting survives: once stats.jsonl
exceeds max_file_mb (default 16, 0 disables), records older than the
retention window fold into cumulative per-account totals in a small sidecar
file (tmp+rename on both) and leave the log. alltime = folded + in-file, a
record is never counted twice, verified end-to-end on synthetic data
(1.2 MB → 301 KB, totals identical before/after compaction and restarts).

Routing: a task rule can now PIN the complexity tier —
`[routing.tasks.legal] models=[...] tier="high"` (old plain-array rules parse
unchanged). The pinned tier replaces the router's verdict before the
escalation check and reports itself as route_source "router+task-rule@high".
The Routing page grows a per-task «Complexity» selector (Auto/low/medium/
high); verified live: a code prompt with a pinned high went
router+task-rule@high → the expensive model.

13 new strings in all 7 languages; docs/ROUTING(.ru) updated.

web/js/i18n/zh.js

@@ -621,4 +621,10 @@ export default {
   "bm.cutByCap": "预算全用于思考——未能给出答案",
   "shell.uiStale": "网关已更新 — 此页面运行的是旧界面。",
   "shell.reload": "重新加载",
+  "keys.col.requests": "请求数",
+  "keys.col.tokens": "Token 入/出",
+  "keys.col.costAll": "$ 累计",
+  "routing.tasks.tier": "复杂度",
+  "routing.tasks.tierAuto": "自动（路由器）",
+  "routing.tasks.tierHint": "为该任务固定复杂度层级：替换路由器的判断，high 仍会越过模型列表进行升级。",
 };