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>

tools/local-dev/README.md

@@ -99,6 +99,16 @@ To disable the bootstrap, clear `BACKEND_DEV_SANDBOX_EMAIL` in
 `tools/local-dev/.env` and `docker compose up -d backend` (or
 `make rebuild`). Existing users / games are not removed.
 
+Terminal sandbox games — anything in `cancelled`, `finished`, or
+`start_failed` — are deleted on every boot before find-or-create
+runs. The cascade declared in `00001_init.sql` removes the
+matching memberships, applications, invites, runtime records,
+and player mappings in the same write, so the dev user's lobby
+shows exactly one running tile at all times. Cancelling the
+sandbox manually and running `docker compose restart backend`
+(or `make rebuild`) yields a fresh game without leaving dead
+tiles behind.
+
 The bootstrap requires:
 - `galaxy-engine:local-dev` Docker image (`make build-engine`).
 - `BACKEND_DEV_SANDBOX_ENGINE_VERSION` parses as plain semver