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/devsandbox/bootstrap_test.go

@@ -80,9 +80,9 @@ func TestBootstrapRejectsMissingDeps(t *testing.T) {
 var errMissingDepsSentinel = errors.New("sentinel")
 
 // TestTerminalSandboxStatus pins the contract that decides whether a
-// previously created sandbox game is reusable. Terminal states force
-// the bootstrap to create a new game on the next boot rather than
-// hand the developer a dead lobby tile.
+// previously created sandbox game gets purged on the next boot.
+// Terminal states are deleted (cascade-style) so the developer's
+// lobby never piles up dead tiles between `make rebuild` cycles.
 func TestTerminalSandboxStatus(t *testing.T) {
 	terminal := []string{"cancelled", "finished", "start_failed"}
 	live := []string{"draft", "enrollment_open", "ready_to_start", "starting", "running", "paused"}