feat(admin): U7 — usage dashboard + quota enforcement
UsageRecord extended with user_id + department_id (backward compatible).
UsageStore Protocol extended: record() accepts user_id/department_id,
get_usage() accepts filters, new get_usage_by_user/department methods.
RedisUsageStore uses versioned keys (v2) for new records.
LLMGateway.chat()/chat_stream() accept user_id, department_ids, db_path.
Quota check before provider call: model whitelist + token limit + cost
limit (daily). Multi-department uses strictest-wins (any exceed → reject).
QuotaExceededError → 429 at route layer.
UsageService: summary, timeseries, by-model, top-users, export (CSV/JSON).
5 new admin endpoints under /admin/usage/*.
llm_gateway.py routes pass DepartmentContext + db_path to gateway,
catch QuotaExceededError → 429 (JSON for /chat, SSE error for /stream).
84 new tests. 441 admin+usage tests pass, no regressions.
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