standardize packaging, tooling, docs, CI, and licensing

mail_intake/models/message.py

@@ -13,7 +13,6 @@ provider-specific fields or semantics.
 
 from dataclasses import dataclass
 from datetime import datetime
-from typing import Optional, Dict
 
 
 @dataclass(frozen=True)
@@ -54,7 +53,7 @@ class MailIntakeMessage:
     Sender email address.
     """
 
-    from_name: Optional[str]
+    from_name: str | None
     """
     Optional human-readable sender name.
     """
@@ -74,7 +73,7 @@ class MailIntakeMessage:
     Short provider-supplied preview snippet of the message.
     """
 
-    raw_headers: Dict[str, str]
+    raw_headers: dict[str, str]
     """
     Normalized mapping of message headers (header name → value).
     """