fix(web): protejeaza decriptarea override_json in preview + inchide 3.6 (CLOSE)

decrypt_creds(override_json) era in afara try/except-ului care protejeaza
raw_json in preview_import (import_router) si _web_compute_preview (routes).
La rotatie cheie Fernet (risc acceptat R4) sau token corupt, raw_json degrada
gratios la {} dar override_json arunca 500 pe tot batch-ul. Acum ambalat
identic (fallback None -> {}).

Prins de /code-review high la CLOSE. Writeback: ROADMAP 3.6 -> DONE,
PRD -> inchis + Raport CLOSE. Duplicare _override_of/canonicalize notata
ca cleanup viitor (disciplina backend-neatins). 523 teste pass.

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

app/api/v1/import_router.py

@@ -696,7 +696,10 @@ def preview_import(
                 rows.append(row_data or {})
             except Exception:
                 rows.append({})
-            ov = decrypt_creds(r["override_json"]) if r["override_json"] else None
+            try:
+                ov = decrypt_creds(r["override_json"]) if r["override_json"] else None
+            except Exception:
+                ov = None
             overrides.append(ov or {})
 
         # Obtine coloanele