feat(5.20): US-012 audit evenimente medii + teste e2e

log_event best-effort (refoloseste conn apelantului, fara PII in context) la:
- rar_env_activat / rar_env_dezactivat: activare/dezactivare mediu in cont_rar_medii
- rar_env_default_schimbat: schimbare efectiva default in cont_rar_medii si in
  toggle-ul din statusbar (fragment_status_toggle_env)
- rar_env_blocat: tinta indisponibila — 422 pe canalul API (router.py) + WARNING
  pe caile de import web (fallback existent neschimbat, doar logging adaugat)

tests/test_e2e_rar_env.py: lant import->queued cu rar_env corect (ambele canale),
activare Productie logata, tinta indisponibila blocata + logata.

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

app/api/v1/router.py

@@ -175,6 +175,14 @@ def create_prezentari(
         try:
             env = rezolva_rar_env(conn, acct, req.rar_env)
         except MediuIndisponibil as e:
+            # US-012: audit blocare mediu indisponibil (tip='rar_env_blocat').
+            log_event(
+                "rar_env_blocat",
+                nivel="WARNING",
+                account_id=acct,
+                context={"env": e.env},
+                conn=conn,
+            )
             raise HTTPException(
                 status_code=422,
                 detail=err_eroare(
@@ -364,6 +372,14 @@ def valideaza_prezentari(
         try:
             env = rezolva_rar_env(conn, acct, req.rar_env)
         except MediuIndisponibil as e:
+            # US-012: audit blocare mediu indisponibil pe dry-run (tip='rar_env_blocat').
+            log_event(
+                "rar_env_blocat",
+                nivel="WARNING",
+                account_id=acct,
+                context={"env": e.env},
+                conn=conn,
+            )
             raise HTTPException(
                 status_code=422,
                 detail=err_eroare(