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_dedup_error.py

@@ -82,16 +82,16 @@ def test_resubmit_actualizeaza_creds_pe_reactivare(client):
     enc_dupa = _creds_enc(sid)
     assert enc_dupa is not None
     assert enc_dupa != enc_initial, "creds-urile trebuie actualizate la reactivare"
-    # propagat si in accounts.rar_creds_enc (canal durabil, decizie #17)
+    # US-013: creds propagate in slotul per-env (rar_creds_test_enc, ancora globala=test)
     from app.db import get_connection
     from app.crypto import decrypt_creds
     conn = get_connection()
     try:
-        row = conn.execute("SELECT rar_creds_enc FROM accounts WHERE id=1").fetchone()
+        row = conn.execute("SELECT rar_creds_test_enc FROM accounts WHERE id=1").fetchone()
     finally:
         conn.close()
-    assert row["rar_creds_enc"] is not None
-    assert decrypt_creds(row["rar_creds_enc"])["password"] == "parolaNoua"
+    assert row["rar_creds_test_enc"] is not None
+    assert decrypt_creds(row["rar_creds_test_enc"])["password"] == "parolaNoua"
 
 
 def test_resubmit_peste_sent_ramane_deduped(client):