feat(regression): add E2E canary regression suite (tests/regression/)

Seven canaries prove both halves of the server's job:
- GREEN: good apps are reported healthy (good-simple + good-significant)
- RED: broken apps are caught at intended tier (false-green guard + 4 per-tier)

Fixtures: custom-html-bkp-bad (backup tier RED) + custom-html-rst-bad (restore tier RED).
All 7 canaries verified on live server (see STATUS-regression.md for artifacts).

Not wired to per-commit CI — run on-demand: pytest -m canary tests/regression/

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/custom-html-rst-bad/recipe_meta.py

@@ -0,0 +1,3 @@
+# custom-html-rst-bad — regression fixture for bad-restore canary.
+# BACKUP_CAPABLE=True forces the backup tier to run even though the recipe has no backupbot label.
+BACKUP_CAPABLE = True