Let only the owner delete a project, not an administrator (#272)

#179 decided that nobody deletes a project on someone else's behalf.
The API still let an administrator do it, because the admin store
treated every project as its own. Option A from the issue is now in
place: opening, editing, snapshots and release stay open to the
administrator, deleting stays with the owner.

- Store::delete checks the owner under the save lock and throws
  NotOwnerException for anyone else. It follows the pattern of
  StaleRevisionException.
- The route answers that with 403 "Only its owner may delete a
  project". A file without an owner is covered the same way.
  Non-admins still get 404 for a project that is not theirs.
- The editor has no way to delete a foreign project, the project
  picker lists only one's own. Nothing changes there.
- An account that owns projects can no longer be deleted by an
  administrator at all. Its owner has to delete them first.
- e2e 49 deleted the throwaway projects as administrator, both in
  its sweep and in 49i. It now signs in as the owner for that.
  49i checks both answers.

Tests:
- tests/php/accounts.test.php: 56 checks passed (was 52, one check
  turned around, five new)
- e2e 49: 22 checks passed (was 21), deleteByAdmin 403,
  deleteByOwner 204
- Gate: 197 files, 2855 tests passed
- Counter-checks: without the owner check 4 PHP checks and 49i fail
- The e2e total of 1603 is computed (1602 + 1), not measured

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -45,7 +45,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1602 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1603 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs