[management] Cache peer snapshot + consolidate auth reads on Sync hot path

Trim the fast-path Sync handler by removing two DB round trips on cache hit:

1. Consolidate GetUserIDByPeerKey + GetAccountIDByPeerPubKey into a single
   GetPeerAuthInfoByPubKey store call. Both looked up the same peer row by
   pubkey and returned one column each; the new method SELECTs both columns
   in one query. AccountManager exposes it as GetPeerAuthInfo.

2. Extend peerSyncEntry with AccountID, PeerID, PeerKey, Ephemeral and a
   HasUser flag so the cache carries everything the fast path needs. On
   cache hit with a matching metaHash:

    - The Sync handler skips GetPeerAuthInfo entirely (entry.AccountID and
      entry.HasUser drive the loginFilter gate).
    - commitFastPath skips GetPeerByPeerPubKey by using the cached peer
      snapshot for OnPeerConnectedWithPeer.

Old cache entries from pre-step-2 shape still decode (missing fields zero
out) but IsComplete() returns false, so they fall through to the slow path
and get rewritten with the full shape on first pass. No migration needed.

Expected impact on a 16.8 s pathological Sync observed in production:
~6 s saved from eliminating one auth-read round trip, the pre-fast-path
GetPeerAuthInfo on cache hit, and GetPeerByPeerPubKey in commitFastPath.
Cache miss / cold start remain on the slow path unchanged.

Account-serial, ExtraSettings and peer-group caching — the remaining
synchronous DB reads — are deliberately left for a follow-up so the
invalidation design can be proven incrementally.

management/server/account/manager_mock.go

@@ -900,6 +900,22 @@ func (mr *MockManagerMockRecorder) GetPeer(ctx, accountID, peerID, userID interf
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetPeer", reflect.TypeOf((*MockManager)(nil).GetPeer), ctx, accountID, peerID, userID)
 }
 
+// GetPeerAuthInfo mocks base method.
+func (m *MockManager) GetPeerAuthInfo(ctx context.Context, peerKey string) (string, string, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetPeerAuthInfo", ctx, peerKey)
+	ret0, _ := ret[0].(string)
+	ret1, _ := ret[1].(string)
+	ret2, _ := ret[2].(error)
+	return ret0, ret1, ret2
+}
+
+// GetPeerAuthInfo indicates an expected call of GetPeerAuthInfo.
+func (mr *MockManagerMockRecorder) GetPeerAuthInfo(ctx, peerKey interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetPeerAuthInfo", reflect.TypeOf((*MockManager)(nil).GetPeerAuthInfo), ctx, peerKey)
+}
+
 // GetPeerGroups mocks base method.
 func (m *MockManager) GetPeerGroups(ctx context.Context, accountID, peerID string) ([]*types.Group, error) {
 	m.ctrl.T.Helper()