fix UI 503s under checkpoint load

- Store: skip identical CT checkpoint writes (CertSpotter calls StoreLogState
  per batch across 64 logs; hashing the state avoids redundant writes).
- webui: cache /api/stats for 10s and raise API query timeouts; UI polls, so
  repeated aggregate queries no longer queue behind checkpoint bursts.
- reword discovery detail (no 'name enumeration' for S3/GCS; only Azure uses a
  container list).

internal/store/store.go

@@ -5,6 +5,7 @@ package store
 import (
 	"context"
 	"fmt"
+	"sync"
 	"time"
 
 	"github.com/jackc/pgx/v5"
@@ -20,6 +21,9 @@ import (
 type Store struct {
 	pool    *pgxpool.Pool
 	metrics *metrics.Metrics
+
+	stateMu   sync.Mutex
+	lastState map[string][32]byte // per-log checkpoint hash; skip identical writes
 }
 
 // Open connects to PostgreSQL. The pool is bounded; the caller configures the
@@ -38,7 +42,7 @@ func Open(ctx context.Context, databaseURL string, maxConns int) (*Store, error)
 	if err != nil {
 		return nil, err
 	}
-	return &Store{pool: pool}, nil
+	return &Store{pool: pool, lastState: make(map[string][32]byte)}, nil
 }
 
 func (s *Store) SetMetrics(m *metrics.Metrics) { s.metrics = m }