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/migrations/20260503110000_add_generation_task_leases.down.sql

@@ -0,0 +1,9 @@
+DROP INDEX IF EXISTS idx_generation_tasks_queued_stale;
+DROP INDEX IF EXISTS idx_generation_tasks_running_lease;
+
+ALTER TABLE generation_tasks
+    DROP COLUMN IF EXISTS last_heartbeat_at,
+    DROP COLUMN IF EXISTS attempt_count,
+    DROP COLUMN IF EXISTS lease_expires_at,
+    DROP COLUMN IF EXISTS lease_owner,
+    DROP COLUMN IF EXISTS lease_token;

server/migrations/20260503110000_add_generation_task_leases.up.sql

@@ -0,0 +1,14 @@
+ALTER TABLE generation_tasks
+    ADD COLUMN lease_token VARCHAR(64),
+    ADD COLUMN lease_owner VARCHAR(128),
+    ADD COLUMN lease_expires_at TIMESTAMPTZ,
+    ADD COLUMN attempt_count INT NOT NULL DEFAULT 0,
+    ADD COLUMN last_heartbeat_at TIMESTAMPTZ;
+
+CREATE INDEX idx_generation_tasks_running_lease
+    ON generation_tasks(lease_expires_at, id)
+    WHERE status = 'running';
+
+CREATE INDEX idx_generation_tasks_queued_stale
+    ON generation_tasks(updated_at, id)
+    WHERE status = 'queued';