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>

app/schema.sql

@@ -19,8 +19,8 @@ CREATE TABLE IF NOT EXISTS accounts (
     -- vezi accounts.delete_account — randul ramane doar pentru audit).
     status          TEXT NOT NULL DEFAULT 'active'
                        CHECK (status IN ('pending','active','blocked','archived','deleted')),
-    rar_creds_enc   TEXT,       -- LEGACY (PRD 5.20 US-013 dropeaza coloana): creds RAR durabile env-less
     -- Medii RAR per cont (PRD 5.20 US-001). Fiecare mediu = bifa de activare + slot creds.
+    -- accounts.rar_creds_enc (legacy env-less) a fost dropata in US-013 (SQLite DROP COLUMN).
     -- medii_disponibile = enabled AND creds prezente (app/rar_env.py). Cont client nou =
     -- Productie on / Testare off (clientii declara real); contul operator se pune manual pe Testare.
     rar_test_enabled INTEGER NOT NULL DEFAULT 0 CHECK (rar_test_enabled IN (0, 1)),