fix(dashboard,worker): vérité produit P0 — dashboard+worker+VWB export

War-room clôture DGX 2026-06-18 (recadrage Dom : graphe/apprentissage/mémoire/dashboard = surface produit P0).
Le dashboard et le statut worker affichaient des états faux ; corrige pour refléter la vérité du produit.

- dashboard FAISS: distingue index brut / metadata HMAC invalide / runtime / absent (plus de faux "inactif")
- dashboard process-mining: 503 explicite missing_dependency (plus de message trompeur)
- dashboard /api/workflows + system/status: lecture DB VWB v3 canonique (total réel = 24, plus de 0)
- worker /processing/status: véridique (lit _worker_health.json) + statut "idle/armé (lazy)" distinct de "dégradé (échec)"
- VWB export: N steps -> N actions/edges (dernière action n'est plus perdue)
- tests: dashboard routes, worker status truthfulness, export VWB

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/integration/test_t2a_extract.py

@@ -296,9 +296,11 @@ def test_export_workflow_with_t2a_chain():
     ]
     core = convert_vwb_to_core_workflow(workflow_data, steps_data)
     edge_types = [e["action"]["type"] for e in core["edges"]]
+    assert len(core["edges"]) == len(steps_data)
     assert "extract_text" in edge_types
     assert "t2a_decision" in edge_types
     assert "pause_for_human" in edge_types
+    assert edge_types[-1] == "mouse_click"
     # Vérifier que le templating est bien transporté
     t2a_edge = next(e for e in core["edges"] if e["action"]["type"] == "t2a_decision")
     assert t2a_edge["action"]["parameters"]["input_template"] == "{{dpi}}"