Implement email-address-based conversation matching

Closes the gap where parents sending a new email (instead of replying)
would lose their registration progress. All changes follow the
email-based-conversation-matching OpenSpec change.

Key changes
-----------
storage/json_store.py
  - normalize_email() helper (lowercase + trim)
  - Conversations now keyed by sender email address, not thread ID
  - Versioned registration storage: data/registrations/<email>/v<N>_<ts>.json
  - current.json always reflects the latest version
  - save_registration() returns (email_key, version) tuple
  - save_registration_version() for updates with change_summary
  - get_registration_history() returns all versions in order

models/conversation.py
  - Added last_inbound_message_id field for reply threading (not matching)

channels/email_channel.py
  - fetch_unread_messages() no longer exposes thread_id
  - Conversation matching removed from channel layer (now in agent)
  - Removed _resolve_thread_id() — threading headers kept for SMTP only

agent/core.py
  - process_message() takes parent_email + inbound_message_id (no thread ID)
  - Looks up conversation by normalized email address
  - Post-completion handler: detects intent (question / update / new_child)
  - Registration updates: diffs old vs new, versions storage, notifies admin

agent/prompts.py
  - build_system_prompt() dispatches to registration or post-completion prompt
  - Post-completion prompt guides LLM to return intent field
  - Reminder language updated: no expiration threats

notifications/notifier.py
  - notify_admin() accepts version parameter
  - notify_registration_update() sends "Registration Updated" emails with diff
  - _build_update_body() includes field-level old→new change summary

main.py
  - Poll loop passes parent_email + inbound_message_id to agent (no thread_id)

https://claude.ai/code/session_01HaUFs7SaLD5SoiuGCY27Tw

src/models/conversation.py

@@ -1,4 +1,4 @@
-"""Conversation state model — persisted per email thread."""
+"""Conversation state model — persisted per sender email address."""
 
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
@@ -20,7 +20,7 @@ class ChatMessage:
 
 @dataclass
 class ConversationState:
-    conversation_id: str
+    conversation_id: str                           # normalized sender email address
     language: str = "de"                           # "de" or "en"
     flow_step: str = "greeting"                    # current step in registration flow
     registration: RegistrationData = field(default_factory=RegistrationData)
@@ -32,6 +32,9 @@ class ConversationState:
     last_activity: str = field(default_factory=_now)
     completed: bool = False
     reminder_count: int = 0
+    # Most recent inbound Message-ID — used for reply threading headers only,
+    # NOT for conversation matching (which is always by email address).
+    last_inbound_message_id: str = ""
 
     def to_dict(self) -> dict:
         return {
@@ -50,6 +53,7 @@ class ConversationState:
             "last_activity": self.last_activity,
             "completed": self.completed,
             "reminder_count": self.reminder_count,
+            "last_inbound_message_id": self.last_inbound_message_id,
         }
 
     @classmethod
@@ -73,4 +77,5 @@ class ConversationState:
         state.last_activity = data.get("last_activity", "")
         state.completed = data.get("completed", False)
         state.reminder_count = data.get("reminder_count", 0)
+        state.last_inbound_message_id = data.get("last_inbound_message_id", "")
         return state