refactor(knowledge): migrate deleted-item cleanup from polling scheduler to event-driven worker

Replaces the scheduler-polled KnowledgeDeletedCleanupWorker with an
event-driven KnowledgeDeletedCleanupEventWorker in tenant-api; cleanup
events are now enqueued transactionally at delete time instead of swept
periodically. Also fixes statement_timeout parameterization in
MonitoringRetentionWorker and propagates errors from cleanupDeletedKnowledgeItem.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

server/migrations/20260520130000_add_knowledge_deleted_cleanup_events.down.sql

@@ -0,0 +1,4 @@
+DROP INDEX IF EXISTS idx_knowledge_deleted_cleanup_processing;
+DROP INDEX IF EXISTS idx_knowledge_deleted_cleanup_pending;
+DROP INDEX IF EXISTS uk_knowledge_deleted_cleanup_open;
+DROP TABLE IF EXISTS knowledge_deleted_cleanup_events;

server/migrations/20260520130000_add_knowledge_deleted_cleanup_events.up.sql

@@ -0,0 +1,28 @@
+CREATE TABLE IF NOT EXISTS knowledge_deleted_cleanup_events (
+    id                BIGSERIAL PRIMARY KEY,
+    tenant_id          BIGINT NOT NULL,
+    knowledge_item_id  BIGINT NOT NULL,
+    status             VARCHAR(20) NOT NULL DEFAULT 'pending',
+    attempt_count      INT NOT NULL DEFAULT 0,
+    available_at       TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    locked_by          TEXT,
+    locked_at          TIMESTAMPTZ,
+    last_error         TEXT,
+    completed_at       TIMESTAMPTZ,
+    created_at         TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at         TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    CHECK (status IN ('pending', 'processing', 'completed', 'failed')),
+    CHECK (attempt_count >= 0)
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS uk_knowledge_deleted_cleanup_open
+    ON knowledge_deleted_cleanup_events (tenant_id, knowledge_item_id)
+    WHERE completed_at IS NULL;
+
+CREATE INDEX IF NOT EXISTS idx_knowledge_deleted_cleanup_pending
+    ON knowledge_deleted_cleanup_events (available_at, id)
+    WHERE completed_at IS NULL AND status IN ('pending', 'failed');
+
+CREATE INDEX IF NOT EXISTS idx_knowledge_deleted_cleanup_processing
+    ON knowledge_deleted_cleanup_events (locked_at, id)
+    WHERE completed_at IS NULL AND status = 'processing';