refactor(V2): IBKR settings — TypedDict return + docstrings

Code review polish:
- credentials() returns IBKRCredentials TypedDict (was bare dict)
- Method docstring matching Deribit pattern
- Inline comment explaining account_id env-only design

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/cerbero_mcp/settings.py

@@ -1,10 +1,22 @@
 """Pydantic Settings: legge .env e variabili d'ambiente."""
 from __future__ import annotations
 
+from typing import TypedDict
+
 from pydantic import Field, SecretStr
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
+class IBKRCredentials(TypedDict):
+    consumer_key: str
+    access_token: str
+    access_token_secret: str
+    signature_key_path: str
+    encryption_key_path: str
+    account_id: str
+    dh_prime: str
+
+
 class _Sub(BaseSettings):
     """Base per sub-settings, condivide model_config con env_file."""
     model_config = SettingsConfigDict(
@@ -111,6 +123,7 @@ class IBKRSettings(_Sub):
     access_token_secret_testnet: SecretStr | None = None
     signature_key_path_testnet: str | None = None
     encryption_key_path_testnet: str | None = None
+    # account_id has no base variant: paper and live accounts are always distinct
     account_id_testnet: str | None = None
 
     consumer_key_live: str | None = None
@@ -128,7 +141,12 @@ class IBKRSettings(_Sub):
     ws_max_subscriptions: int = 80
     ws_idle_timeout_s: int = 300
 
-    def credentials(self, env: str) -> dict:
+    def credentials(self, env: str) -> IBKRCredentials:
+        """Return credential dict for given env.
+        Prefers env-specific (_TESTNET / _LIVE) values; falls back to base
+        (IBKR_CONSUMER_KEY etc.) for legacy single-pair setups.
+        ValueError if any required field missing.
+        """
         if env == "testnet":
             ck = self.consumer_key_testnet or self.consumer_key
             at = self.access_token_testnet or self.access_token