feat(5.20): US-013 retragere accounts.rar_creds_enc -> per-env + DROP cu garda

Toate citirile pe coloana legacy accounts.rar_creds_enc mutate pe sloturile
per-env (rar_creds_test_enc/rar_creds_prod_enc): worker fallback+keepalive,
are_creds (web) si are_creds_rar (integrare, +are_creds_test/_prod), write-back
API la reactivare, purjare la stergere cont, _get_acasa_context/_fetch_cont_env_state.

Contract API (aditiv): POST /v1/conturi/rar-creds primeste rar_target optional
(test/prod), scrie in slotul corect + activeaza mediul; DELETE primeste ?env
(sterge un slot sau ambele). Documentat in docs/api-rar-contract.md.

DROP cu garda in db.py (schema.sql fara coloana pe DB fresh):
- 6a: eliminat ADD COLUMN rar_creds_enc (fara ping-pong re-ADD dupa DROP)
- 6b: try/except fail-safe (nu crapa boot-ul) + garda sqlite_version >= 3.35
- 6c: re-backfill old->new imediat inainte de assert (ancora globala)
- garda orfane: DROP anulat daca vreun creds legacy nu a aterizat in slot per-env
- backup criptat accounts_rar_creds_enc_backup inainte de DROP
- 6d: verificare prin PRAGMA table_info (NU grep — submissions are aceeasi coloana)
Garda one-way, idempotenta la boot repetat (verificat). submissions.rar_creds_enc
ramane neatinsa.

tests/test_retragere_creds_enc.py: niciun read pe coloana veche, conturi rar-creds
env-aware, are_creds per-env, DROP blocat de garda la lipsa copiere. 9 teste
existente actualizate pe sloturi per-env.

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

tests/test_foundation.py

@@ -34,9 +34,14 @@ def _tables(conn) -> set[str]:
 
 # --- Coloane noi ---
 
-def test_accounts_rar_creds_enc(db_conn):
-    cols = _table_cols(db_conn, "accounts")
-    assert "rar_creds_enc" in cols
+def test_accounts_rar_creds_enc_dropata(db_conn):
+    """US-013: accounts.rar_creds_enc a fost dropata; submissions.rar_creds_enc ramane."""
+    acc_cols = _table_cols(db_conn, "accounts")
+    sub_cols = _table_cols(db_conn, "submissions")
+    assert "rar_creds_enc" not in acc_cols, \
+        "accounts.rar_creds_enc trebuie sa fie ABSENTA dupa US-013 DROP"
+    assert "rar_creds_enc" in sub_cols, \
+        "submissions.rar_creds_enc trebuie sa RAMANA (creds efemere per-cerere)"
 
 
 def test_submissions_batch_id(db_conn):
@@ -153,7 +158,8 @@ def test_migrate_on_existing_db(monkeypatch):
     assert "batch_id" in sub_cols
     assert "row_index" in sub_cols
     assert "purge_after" in sub_cols
-    assert "rar_creds_enc" in acc_cols
+    # US-013: accounts.rar_creds_enc a fost dropata.
+    assert "rar_creds_enc" not in acc_cols
     conn.close()
     get_settings.cache_clear()