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.

config/gateway.example.toml

@@ -119,6 +119,10 @@ enabled   = true
 file      = "config/stats.jsonl"   # empty = in-memory only
 ring_size = 500                     # how many recent requests to keep in the feed
 retention = "7d"                    # aggregate/series depth (replayed on startup)
+# stats.jsonl is compacted once it exceeds this size: old records fold into
+# cumulative per-account totals (sidecar file), so per-key accounting survives
+# while the disk stays bounded. 0 disables.
+max_file_mb = 16
 
 # ============================================================================
 # Semantic cache — returns a cached answer for prompts semantically near a