local-dev: auto-purge terminal Dev Sandbox games on every boot

Previously a cancelled / finished / start_failed sandbox game would
hang in the dev user's lobby until manually cleaned up — `make up`
would create a new running game alongside it but the dead tiles
piled up. Now backend's `devsandbox.Bootstrap` deletes every
terminal sandbox game owned by the dev user before find-or-create
runs, so the lobby always shows exactly one running tile.

Schema: `runtime_records` and `player_mappings` gain
`ON DELETE CASCADE` on their `game_id` foreign keys so a single
`DELETE FROM games` cleans every referencing row in one write.
Pre-prod migration rule applies — change goes into
`00001_init.sql`, not a new migration.

API: `lobby.Service.DeleteGame` is the new destructive helper that
backs the bootstrap purge. It bypasses the cancel-cascade-notify
pipeline; production callers must stay on the regular lifecycle.
The dev-sandbox docs in `tools/local-dev/README.md` spell out the
new behaviour.

Tests:
- backend/internal/lobby/lobby_e2e_test.go gains
  `TestDeleteGameCascadesEverything` proving CASCADE works
  end-to-end against a real Postgres testcontainer.
- backend/internal/devsandbox keeps its existing terminal-status
  contract test; the new `purgeTerminalSandboxGames` helper rides
  on the same `terminalSandboxStatus` predicate.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/lobby/store.go

@@ -232,6 +232,22 @@ func (s *Store) ListMyGames(ctx context.Context, userID uuid.UUID) ([]GameRecord
 	return modelsToGameRecords(rows)
 }
 
+// DeleteGame removes the row at gameID. Cascades through every
+// referencing table (memberships / applications / invites /
+// runtime_records / player_mappings — all declared with ON DELETE
+// CASCADE in `00001_init.sql`). Idempotent: returns nil when no row
+// matches. Used by the dev-sandbox bootstrap to scrub terminal
+// games on every backend boot so the developer's lobby never piles
+// up cancelled tiles.
+func (s *Store) DeleteGame(ctx context.Context, gameID uuid.UUID) error {
+	g := table.Games
+	stmt := g.DELETE().WHERE(g.GameID.EQ(postgres.UUID(gameID)))
+	if _, err := stmt.ExecContext(ctx, s.db); err != nil {
+		return fmt.Errorf("lobby store: delete game %s: %w", gameID, err)
+	}
+	return nil
+}
+
 // gameUpdate is the parameter struct for UpdateGame. Nil pointers leave
 // the corresponding column alone.
 type gameUpdate struct {