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/postgres/000023_pg_stat_statements.down.sql

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+DROP EXTENSION IF EXISTS pg_stat_statements;

migrations/postgres/000023_pg_stat_statements.up.sql

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+-- pg_stat_statements requires shared_preload_libraries on the server; extension may fail on dev without restart.
+CREATE EXTENSION IF NOT EXISTS pg_stat_statements;

migrations/postgres/000024_postgres_monitoring.down.sql

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+DROP TABLE IF EXISTS postgres_maintenance_audit;
+DROP TABLE IF EXISTS postgres_monitor_snapshot;

migrations/postgres/000024_postgres_monitoring.up.sql

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+CREATE TABLE IF NOT EXISTS postgres_monitor_snapshot (
+    id TEXT PRIMARY KEY,
+    collected_at TIMESTAMPTZ NOT NULL,
+    payload_json JSONB NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_postgres_monitor_snapshot_collected
+    ON postgres_monitor_snapshot (collected_at DESC);
+
+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 BOOLEAN NOT NULL DEFAULT false,
+    status TEXT NOT NULL,
+    detail_json JSONB,
+    error_message TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    started_at TIMESTAMPTZ,
+    finished_at TIMESTAMPTZ,
+    CONSTRAINT postgres_maintenance_audit_status_chk CHECK (
+        status IN ('queued', 'running', 'succeeded', 'failed', 'cancelled')
+    )
+);
+
+CREATE INDEX IF NOT EXISTS idx_postgres_maintenance_audit_created
+    ON postgres_maintenance_audit (created_at DESC);