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/engine/engine.go

@@ -97,6 +97,9 @@ var (
 	// ErrUnknownVersion is returned when no dictionary is registered for a
 	// (variant, version) pair.
 	ErrUnknownVersion = errors.New("engine: unknown dictionary version")
+	// ErrUnknownDropoutTiles is returned by ParseDropoutTiles for a label that is
+	// neither "remove" nor "return".
+	ErrUnknownDropoutTiles = errors.New("engine: unknown drop-out tile disposition")
 	// ErrIllegalPlay wraps a solver validation failure: off-board geometry, a
 	// word absent from the dictionary, or a play that does not connect.
 	ErrIllegalPlay = errors.New("engine: illegal play")