refactor(httpapi): readiness ping via store backend

ARCH-09: Ping на store.Backend; readiness без прямого pgxpool.Ping в handler.
Co-authored-by: Cursor <[email protected]>

internal/store/backend.go

@@ -1,6 +1,9 @@
 package store
 
-import "time"
+import (
+	"context"
+	"time"
+)
 
 // Backend is the persistence abstraction for the control plane (memory, PostgreSQL, SQLite).
 type Backend interface {
@@ -90,6 +93,9 @@ type Backend interface {
 	// ASNPrefixCache stores RIPEstat announced-prefixes per ASN (global TTL cache).
 	GetASNPrefixCache(asn int64) (*ASNPrefixCacheEntry, bool, error)
 	SetASNPrefixCache(asn int64, holder string, prefixes []string) error
+
+	// Ping verifies backend connectivity (no-op for in-memory).
+	Ping(ctx context.Context) error
 }
 
 // ASNPrefixCacheEntry is a cached RIPEstat response for one ASN.