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/service.go

@@ -0,0 +1,90 @@
+package pgmonitor
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// Service provides PostgreSQL observability and maintenance helpers (control plane instance scope).
+type Service struct {
+	pool  *pgxpool.Pool
+	cache *ttlCache
+}
+
+// NewService constructs a metrics service for the API PostgreSQL pool.
+func NewService(pool *pgxpool.Pool) *Service {
+	if pool == nil {
+		return nil
+	}
+	return &Service{
+		pool:  pool,
+		cache: newTTLCache(10 * time.Second),
+	}
+}
+
+// Pool exposes the underlying pool for job workers.
+func (s *Service) Pool() *pgxpool.Pool {
+	if s == nil {
+		return nil
+	}
+	return s.pool
+}
+
+// Overview returns cached instance-level stats.
+func (s *Service) Overview(ctx context.Context) (Overview, error) {
+	if s == nil || s.pool == nil {
+		return Overview{}, errors.New("pgmonitor: postgres not configured")
+	}
+	if v, ok := s.cache.get("overview"); ok {
+		if o, ok := v.(Overview); ok {
+			return o, nil
+		}
+	}
+	o, err := s.fetchOverview(ctx)
+	if err != nil {
+		return Overview{}, err
+	}
+	s.cache.set("overview", o)
+	return o, nil
+}
+
+// Locks returns active / blocking locks.
+func (s *Service) Locks(ctx context.Context) ([]LockRow, error) {
+	if s == nil || s.pool == nil {
+		return nil, errors.New("pgmonitor: postgres not configured")
+	}
+	if v, ok := s.cache.get("locks"); ok {
+		if rows, ok := v.([]LockRow); ok {
+			return rows, nil
+		}
+	}
+	rows, err := queryLocks(ctx, s.pool)
+	if err != nil {
+		return nil, err
+	}
+	s.cache.set("locks", rows)
+	return rows, nil
+}
+
+// Tables returns top tables by size with I/O stats.
+func (s *Service) Tables(ctx context.Context, limit int) ([]TableStat, error) {
+	if s == nil || s.pool == nil {
+		return nil, errors.New("pgmonitor: postgres not configured")
+	}
+	key := fmt.Sprintf("tables:%d", limit)
+	if v, ok := s.cache.get(key); ok {
+		if rows, ok := v.([]TableStat); ok {
+			return rows, nil
+		}
+	}
+	rows, err := queryTables(ctx, s.pool, limit)
+	if err != nil {
+		return nil, err
+	}
+	s.cache.set(key, rows)
+	return rows, nil
+}