feat(cache,worker): overhaul cache layer and add generation task lease recovery

- shared/cache: add Options/L1/async/metrics/prefix decorators, multi-key ops, Redis pool tuning, and JSON readthrough metrics
- worker-generate: claim tasks via DB lease + heartbeat, requeue stale queued tasks, expire dead leases with refund/cache invalidation
- tenant: version article cache keys so worker recovery invalidations propagate cleanly
- shared/config: expand Redis (pool/timeouts/TLS) and Generation (lease/recovery) configs with defaults

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/tenant/repository/queries/audit.sql

@@ -40,5 +40,10 @@ WHERE tenant_id = sqlc.arg(tenant_id)
 
 -- name: UpdateGenerationTaskStatus :exec
 UPDATE generation_tasks SET status = sqlc.arg(status), error_message = sqlc.arg(error_message),
-       started_at = sqlc.arg(started_at), completed_at = sqlc.arg(completed_at), updated_at = NOW()
+       started_at = sqlc.arg(started_at),
+       completed_at = sqlc.arg(completed_at),
+       lease_token = CASE WHEN sqlc.arg(status)::text IN ('completed', 'failed') THEN NULL ELSE lease_token END,
+       lease_owner = CASE WHEN sqlc.arg(status)::text IN ('completed', 'failed') THEN NULL ELSE lease_owner END,
+       lease_expires_at = CASE WHEN sqlc.arg(status)::text IN ('completed', 'failed') THEN NULL ELSE lease_expires_at END,
+       updated_at = NOW()
 WHERE id = sqlc.arg(id) AND tenant_id = sqlc.arg(tenant_id);