fix(bookstack-analytics): fix concurrent-write race condition and show unique users

[amrit110]

Summary

• Race condition fix: The GCS activity log used a raw read-modify-write pattern with no locking — two simultaneous queries from different users could overwrite each other's entry. Fixed with a compare-and-swap loop using GCS object generation preconditions (if_generation_match). On an HTTP 412 conflict, the logger reloads the latest log and retries up to 5 times.
• Unique Users metric: Replaced the "Unique Sessions" dashboard card (which was nearly equal to total queries since every page load creates a new session ID) with a "Unique Users" metric computed from distinct user_email values. Currently shows 5 unique users vs the misleading 25 sessions.

Root cause investigation

Confirmed via gsutil cat on the live GCS bucket: the raw data IS stored correctly (different users asking the same question produce separate entries with distinct session_id, user_email, and timestamp). The issues were:

1. Concurrent writes could silently drop entries (race condition in activity_logger.py)
2. The dashboard's "Unique Sessions" metric was misleading — it counted UUID session IDs (one per page load), not distinct users

Changes

File	Change
src/aieng_bot/bookstack/activity_logger.py	CAS retry loop, _load_activity_log returns (data, generation), _save_activity_log takes if_generation_match
tests/bookstack/test_activity_logger.py	Updated tests + 3 new tests for CAS retry, exhaustion, and PreconditionFailed re-raise
bookstack_agent/ui/lib/bookstack-types.ts	Add unique_users: number to BookstackMetrics
bookstack_agent/ui/lib/bookstack-data-fetcher.ts	Compute unique_users from distinct non-null user_email values
bookstack_agent/ui/app/analytics/components/query-metrics.tsx	Replace "Unique Sessions" card with "Unique Users"

Test plan

• All 250 existing tests pass (uv run pytest)
• 3 new tests: CAS conflict retry, retry exhaustion, PreconditionFailed re-raise
• TypeScript type-checks clean (tsc --noEmit)
• Linting clean (ruff check)