docs(mail_intake): add comprehensive docstrings across ingestion, adapters, auth, and parsing layers

- docs(mail_intake/__init__.py): document module-based public API and usage patterns
- docs(mail_intake/ingestion/reader.py): document high-level ingestion orchestration
- docs(mail_intake/adapters/base.py): document adapter contract for mail providers
- docs(mail_intake/adapters/gmail.py): document Gmail adapter implementation and constraints
- docs(mail_intake/auth/base.py): document authentication provider contract
- docs(mail_intake/auth/google.py): document Google OAuth authentication provider
- docs(mail_intake/models/message.py): document canonical email message model
- docs(mail_intake/models/thread.py): document canonical email thread model
- docs(mail_intake/parsers/body.py): document message body extraction logic
- docs(mail_intake/parsers/headers.py): document message header normalization utilities
- docs(mail_intake/parsers/subject.py): document subject normalization utilities
- docs(mail_intake/config.py): document global configuration model
- docs(mail_intake/exceptions.py): document library exception hierarchy

mail_intake/parsers/subject.py

@@ -1,7 +1,18 @@
+"""
+Subject line normalization utilities for Mail Intake.
+
+This module provides helper functions for normalizing email subject lines
+to enable reliable thread-level comparison and grouping.
+
+Normalization is intentionally conservative to avoid altering semantic
+meaning while removing common reply and forward prefixes.
+"""
+
 import re
 
 
 _PREFIX_RE = re.compile(r"^(re|fw|fwd)\s*:\s*", re.IGNORECASE)
+"""Regular expression matching common reply/forward subject prefixes."""
 
 
 def normalize_subject(subject: str) -> str:
@@ -9,11 +20,19 @@ def normalize_subject(subject: str) -> str:
     Normalize an email subject for thread-level comparison.
 
     Operations:
-    - Strip common prefixes (Re:, Fwd:, FW:)
-    - Collapse whitespace
-    - Preserve original casing (no lowercasing)
+    - Strips common prefixes such as ``Re:``, ``Fwd:``, and ``FW:``
+    - Repeats prefix stripping to handle stacked prefixes
+    - Collapses excessive whitespace
+    - Preserves original casing (no lowercasing)
 
-    This function is intentionally conservative.
+    This function is intentionally conservative and avoids aggressive
+    transformations that could alter the semantic meaning of the subject.
+
+    Args:
+        subject: Raw subject line from a message header.
+
+    Returns:
+        Normalized subject string suitable for thread grouping.
     """
     if not subject:
         return ""