feat(api): scope pe cont la GET-urile de listare /v1/* (PRD 3.2)

Inchide scurgerea cross-account pe GET /v1/prezentari(/{id}),
/v1/mapari(/pending) si /v1/audit/export. Toate primesc
Depends(resolve_account_id) + account_scope_clause (regula NULL->cont 1,
OV-2). Nomenclatorul ramane global (referinta partajata, fara PII).

- B3: 404 cross-account byte-identic cu 404 inexistent (fara oracol enumerare)
- B4: get_prezentare cu allowlist de campuri (nu mai expune rar_creds_enc/
  payload_json/idempotency_key/rar_error)
- B1: pending_unmapped filtreaza in SQL; default None = global doar pentru web
- B2: helper account_scope_clause (DRY, doar pe submissions nullable)
- B5: index idx_submissions_account_status
- B8: regula de scope documentata in api-rar-contract.md
- TD-3.2: ?account_id != contul cheii -> 400

14 teste noi (cross-account, legacy NULL, B3, B4); suita 313 passed.

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

app/db.py

@@ -77,6 +77,11 @@ def _migrate(conn: sqlite3.Connection) -> None:
             "CREATE INDEX IF NOT EXISTS idx_submissions_batch ON submissions(batch_id) "
             "WHERE batch_id IS NOT NULL"
         )
+    if "idx_submissions_account_status" not in existing_idx:
+        conn.execute(
+            "CREATE INDEX IF NOT EXISTS idx_submissions_account_status "
+            "ON submissions(account_id, status)"
+        )
 
 
 def _now_iso() -> str: