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.

internal/pgmonitor/cache.go

@@ -0,0 +1,37 @@
+package pgmonitor
+
+import (
+	"sync"
+	"time"
+)
+
+type cacheEntry struct {
+	at   time.Time
+	data any
+}
+
+type ttlCache struct {
+	mu    sync.RWMutex
+	ttl   time.Duration
+	items map[string]cacheEntry
+}
+
+func newTTLCache(ttl time.Duration) *ttlCache {
+	return &ttlCache{ttl: ttl, items: make(map[string]cacheEntry)}
+}
+
+func (c *ttlCache) get(key string) (any, bool) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	e, ok := c.items[key]
+	if !ok || time.Since(e.at) > c.ttl {
+		return nil, false
+	}
+	return e.data, true
+}
+
+func (c *ttlCache) set(key string, data any) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.items[key] = cacheEntry{at: time.Now().UTC(), data: data}
+}