feat(db): implement PostgreSQL monitoring and maintenance features

Added PostgreSQL monitoring and maintenance capabilities to the API, including new endpoints for instance-level metrics, maintenance operations, and job scheduling. Updated the HTTP API to support PostgreSQL monitoring routes and integrated a background scheduler for metrics collection. Enhanced the CLI with database commands for maintenance tasks. Updated documentation to reflect these changes.

migrations/sqlite/000024_postgres_monitoring.up.sql

@@ -0,0 +1,20 @@
+CREATE TABLE IF NOT EXISTS postgres_monitor_snapshot (
+    id TEXT PRIMARY KEY,
+    collected_at TEXT NOT NULL,
+    payload_json TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS postgres_maintenance_audit (
+    id TEXT PRIMARY KEY,
+    tenant_id TEXT,
+    actor_prefix TEXT,
+    kind TEXT NOT NULL,
+    target_table TEXT,
+    dry_run INTEGER NOT NULL DEFAULT 0,
+    status TEXT NOT NULL,
+    detail_json TEXT,
+    error_message TEXT,
+    created_at TEXT NOT NULL,
+    started_at TEXT,
+    finished_at TEXT
+);