Stage 4: lobby & social (matchmaking, friends, blocks, chat+nudge, invitations, profile, email, multi-player drop-out)

Engine: multi-player drop-out-and-continue with a per-game tile disposition (remove default / return), resigned seats skipped and excluded from the win, leaver rack never revealed; 2-player behaviour unchanged.

New domains (service/store, no HTTP yet): internal/social (friend request/accept graph, per-user blocks, per-game chat with nudge as a message kind, content filter via mvdan.cc/xurls/v2 + leet/separator normaliser + phone heuristic) and internal/lobby (in-memory variant-keyed matchmaking pool, friend-game invitations invite->accept with lazy 7-day expiry). account gains profile editing and the email confirm-code flow (Mailer seam: SMTP or log mailer).

Migration 00003_social.sql + regenerated jet. main wires the new services into the server (accessors for the Stage 6 handlers); robot substitution stays in Stage 5, REST/stream/push in Stage 6/8. Docs (PLAN, ARCHITECTURE, FUNCTIONAL+ru, TESTING, README) updated.

backend/internal/lobby/lobby.go

@@ -0,0 +1,61 @@
+// Package lobby forms games: an in-memory matchmaking pool that pairs two humans
+// for an auto-match, and friend-game invitations (invite -> accept) that start a
+// 2-4 player game once every invitee has accepted. Both produce a game through the
+// game domain (a GameCreator); neither imports the engine. The matchmaking pool
+// is in-memory and lost on restart (players re-queue); the robot that substitutes
+// for a missing human after a short wait is added in a later stage.
+package lobby
+
+import (
+	"context"
+	"errors"
+
+	"github.com/google/uuid"
+
+	"scrabble/backend/internal/game"
+)
+
+// GameCreator is the slice of the game domain the lobby needs: starting a seated
+// game. game.Service satisfies it.
+type GameCreator interface {
+	Create(ctx context.Context, params game.CreateParams) (game.Game, error)
+}
+
+// Blocker reports whether two accounts have a block between them (either
+// direction). social.Service satisfies it; the lobby uses it to refuse
+// invitations between blocked accounts.
+type Blocker interface {
+	IsBlocked(ctx context.Context, a, b uuid.UUID) (bool, error)
+}
+
+// Auto-match defaults: a casual two-player game on the longest move clock with one
+// hint per player (docs/ARCHITECTURE.md §6). The drop-out tile disposition is moot
+// for two players, so the engine default (remove) applies.
+const (
+	autoMatchHintsAllowed   = true
+	autoMatchHintsPerPlayer = 1
+)
+
+// Sentinel errors returned by the lobby.
+var (
+	// ErrAlreadyQueued is returned when an account already waits in a pool.
+	ErrAlreadyQueued = errors.New("lobby: account already in the matchmaking pool")
+	// ErrInvalidInvitation is returned for a malformed invitation (bad player
+	// count, duplicate or self invitee, or unacceptable settings).
+	ErrInvalidInvitation = errors.New("lobby: invalid invitation")
+	// ErrInvitationBlocked is returned when a block stands between the inviter and
+	// an invitee.
+	ErrInvitationBlocked = errors.New("lobby: invitation blocked between accounts")
+	// ErrInvitationNotFound is returned when no invitation matches the lookup.
+	ErrInvitationNotFound = errors.New("lobby: invitation not found")
+	// ErrInvitationNotPending is returned when an invitation is no longer open.
+	ErrInvitationNotPending = errors.New("lobby: invitation is not pending")
+	// ErrInvitationExpired is returned when an invitation has passed its deadline.
+	ErrInvitationExpired = errors.New("lobby: invitation has expired")
+	// ErrNotInvited is returned when an account is not an invitee of the invitation.
+	ErrNotInvited = errors.New("lobby: account was not invited")
+	// ErrAlreadyResponded is returned when an invitee has already accepted or declined.
+	ErrAlreadyResponded = errors.New("lobby: invitee has already responded")
+	// ErrNotInviter is returned when a non-inviter tries to cancel an invitation.
+	ErrNotInviter = errors.New("lobby: only the inviter may cancel")
+)