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/postgres/migrations/00001_init.sql

@@ -418,7 +418,7 @@ CREATE INDEX race_names_pending_eligible_idx
 -- finished) and the container-state escape hatch (removed) used by
 -- reconciliation when the recorded container has disappeared.
 CREATE TABLE runtime_records (
-    game_id                  uuid        PRIMARY KEY,
+    game_id                  uuid        PRIMARY KEY REFERENCES games (game_id) ON DELETE CASCADE,
     status                   text        NOT NULL,
     current_container_id     text,
     current_image_ref        text,
@@ -465,7 +465,7 @@ CREATE TABLE engine_versions (
 -- roster reads. The partial UNIQUE on (game_id, race_name) enforces the
 -- one-race-per-game invariant at the storage boundary.
 CREATE TABLE player_mappings (
-    game_id             uuid        NOT NULL,
+    game_id             uuid        NOT NULL REFERENCES games (game_id) ON DELETE CASCADE,
     user_id             uuid        NOT NULL,
     race_name           text        NOT NULL,
     engine_player_uuid  uuid        NOT NULL,