feat(social): asymmetric per-user block, in-game block control, admin lists

Make a per-user block one-directional and non-destructive: the blocker stops
receiving everything from the blocked user (chat, nudge, friend requests,
invitations) and the matchmaker never pairs them, while the blocked user
notices nothing — their sends still persist by the normal rules but are never
delivered or surfaced (born-read). A block no longer deletes the friendship
(an unblock cleanly restores it) and instant-reads any unread the blocked user
had left for the blocker.

- backend: a directional blockExists guard across chat/nudge/friends/invitations
  (store-but-hide for the blocked->blocker direction, refuse blocker->blocked);
  the matchmaker excludes a block-related pair (both directions) from auto-match;
  user_blocked/user_unblocked notifications to the blocker only (in-app only).
- ui: the opponent score card gains a block ✖️ control mirroring add-friend
  (red "Block?" confirm, mutual-hide while confirming, struck name, hidden chat
  composer when blocked); optimistic apply + event confirm + rollback for both.
- admin: the user card gains cross-linked blocks / blocked-by / friends lists.
- docs: FUNCTIONAL(+ru), ARCHITECTURE §10 + decision record, UI_DESIGN, PRERELEASE.

backend/internal/notify/notify.go

@@ -62,6 +62,16 @@ const (
 	// it re-fetches profile.get to show or hide the banner. It carries no payload (the
 	// banner set rides the profile response). In-app only.
 	NotifyBanner = "banner"
+	// NotifyUserBlocked confirms to the blocker that a per-user block took effect,
+	// carrying the blocked account, so every one of the blocker's sessions updates the
+	// in-game block/add-friend controls and the struck name in place. It is delivered
+	// only to the blocker — never to the blocked user, who must not learn of the block —
+	// and is in-app only (no out-of-app push: blocking is the viewer's own action).
+	NotifyUserBlocked = "user_blocked"
+	// NotifyUserUnblocked confirms an unblock to the (former) blocker, carrying the
+	// unblocked account, so their open game screens restore the controls and un-strike
+	// the name in place. Like NotifyUserBlocked it reaches only the actor and is in-app only.
+	NotifyUserUnblocked = "user_unblocked"
 )
 
 // Intent is one live event destined for a single user. Payload is the