feat: implement chat persistence with MongoDB

- Add full chat persistence to database (conversations and messages)
- Implement MongoChatRepository with cursor-based pagination
- Add getConversations/getConversation endpoints in ChatController
- Save user and assistant messages in ChatService.processMessage()
- Track respondedAction (confirm/reject) on proposed event messages
- Load existing messages on chat screen mount
- Add addMessages() bulk action and chatMessageToMessageData() helper to ChatStore
- Add RespondedAction type and UpdateMessageDTO to shared types

CLAUDE.md

@@ -91,7 +91,7 @@ src/
 └── stores/                     # Zustand state management
     ├── index.ts                # Re-exports all stores
     ├── AuthStore.ts            # user, token, isAuthenticated, login(), logout(), setToken()
-    ├── ChatStore.ts            # messages[], addMessage(), updateMessage(), clearMessages()
+    ├── ChatStore.ts            # messages[], addMessage(), addMessages(), updateMessage(), clearMessages(), chatMessageToMessageData()
     └── EventsStore.ts          # events[], setEvents(), addEvent(), updateEvent(), deleteEvent()
 ```
 
@@ -172,7 +172,7 @@ src/
 │   ├── CalendarEvent.ts  # CalendarEvent, CreateEventDTO, UpdateEventDTO, ExpandedEvent
 │   ├── ChatMessage.ts    # ChatMessage, Conversation, SendMessageDTO, CreateMessageDTO,
 │   │                     # GetMessagesOptions, ChatResponse, ConversationSummary,
-│   │                     # ProposedEventChange, EventAction
+│   │                     # ProposedEventChange, EventAction, RespondedAction, UpdateMessageDTO
 │   └── Constants.ts      # DAYS, MONTHS, Day, Month, DAY_INDEX, DAY_INDEX_TO_DAY,
 │                         # DAY_TO_GERMAN, DAY_TO_GERMAN_SHORT, MONTH_TO_GERMAN
 └── utils/
@@ -184,12 +184,14 @@ src/
 - `User`: id, email, displayName, passwordHash?, createdAt?, updatedAt?
 - `CalendarEvent`: id, userId, title, description?, startTime, endTime, note?, isRecurring?, recurrenceRule?
 - `ExpandedEvent`: Extends CalendarEvent with occurrenceStart, occurrenceEnd (for recurring event instances)
-- `ChatMessage`: id, conversationId, sender ('user' | 'assistant'), content, proposedChange?
+- `ChatMessage`: id, conversationId, sender ('user' | 'assistant'), content, proposedChange?, respondedAction?
 - `ProposedEventChange`: action ('create' | 'update' | 'delete'), eventId?, event?, updates?
 - `Conversation`: id, userId, createdAt?, updatedAt? (messages loaded separately via lazy loading)
 - `CreateEventDTO`: Used for creating events AND for AI-proposed events
 - `GetMessagesOptions`: Cursor-based pagination with `before?: string` and `limit?: number`
 - `ConversationSummary`: id, lastMessage?, createdAt? (for conversation list)
+- `UpdateMessageDTO`: respondedAction? (for marking messages as confirmed/rejected)
+- `RespondedAction`: 'confirm' | 'reject' (tracks user response to proposed events)
 - `Day`: "Monday" | "Tuesday" | ... | "Sunday"
 - `Month`: "January" | "February" | ... | "December"
 
@@ -281,12 +283,14 @@ MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
   - `EventService`: Full CRUD with recurring event expansion via recurrenceExpander
   - `utils/eventFormatters`: getWeeksOverview(), getMonthOverview() with German localization
   - `utils/recurrenceExpander`: expandRecurringEvents() using rrule library for RRULE parsing
+  - `ChatController`: getConversations(), getConversation() with cursor-based pagination support
+  - `ChatService`: getConversations(), getConversation(), processMessage() now persists user/assistant messages to DB, confirmEvent()/rejectEvent() update respondedAction and persist response messages
+  - `MongoChatRepository`: Full CRUD implemented (getConversationsByUser, createConversation, getMessages with cursor pagination, createMessage, updateMessage)
+  - `ChatRepository` interface: updateMessage() added for respondedAction tracking
 - **Stubbed (TODO):**
   - `AuthMiddleware.authenticate()`: Currently uses fake user for testing
   - `AuthController`: refresh(), logout()
   - `AuthService`: refreshToken()
-  - `ChatController`: getConversations(), getConversation()
-  - `MongoChatRepository`: Database persistence for chat
 - **Not started:**
   - `ClaudeAdapter` (AI integration - currently using test responses)
 
@@ -302,20 +306,22 @@ MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
   - Tap-to-open modal overlay showing EventCards for selected day
   - Supports events from adjacent months visible in grid
   - Uses `useFocusEffect` for automatic reload on tab focus
-- Chat screen functional with FlashList, message sending, and event confirm/reject
-  - Messages persisted via ChatStore (survives tab switches)
+- Chat screen fully functional with FlashList, message sending, and event confirm/reject
+  - Messages persisted to database via ChatService and loaded on mount
+  - Tracks conversationId for message continuity across sessions
+  - ChatStore with addMessages() for bulk loading, chatMessageToMessageData() helper
   - KeyboardAvoidingView for proper keyboard handling (iOS padding, Android height)
   - Auto-scroll to end on new messages and keyboard show
   - keyboardDismissMode="interactive" and keyboardShouldPersistTaps="handled"
 - `ApiClient`: get(), post(), put(), delete() implemented
 - `EventService`: getAll(), getById(), getByDateRange(), create(), update(), delete() - fully implemented
-- `ChatService`: sendMessage(), confirmEvent(convId, msgId, action, event?, eventId?, updates?), rejectEvent() - supports create/update/delete actions
+- `ChatService`: sendMessage(), confirmEvent(), rejectEvent(), getConversations(), getConversation() - fully implemented with cursor pagination
 - `EventCardBase`: Shared base component with event layout (header, date/time/recurring icons, description) - used by both EventCard and ProposedEventCard
 - `EventCard`: Uses EventCardBase + edit/delete buttons for calendar display
 - `ProposedEventCard`: Uses EventCardBase + confirm/reject buttons for chat proposals (supports create/update/delete actions)
 - `Themes.tsx`: Centralized color definitions including textPrimary, borderPrimary, eventIndicator, secondaryBg
 - `EventsStore`: Zustand store with setEvents(), addEvent(), updateEvent(), deleteEvent() - stores ExpandedEvent[]
-- `ChatStore`: Zustand store with addMessage(), updateMessage(), clearMessages() - persists messages across tab switches
+- `ChatStore`: Zustand store with addMessage(), addMessages(), updateMessage(), clearMessages() - loads from server on mount and persists across tab switches
 - Auth screens (Login, Register), Event Detail, and Note screens exist as skeletons
 - AuthStore defined with `throw new Error('Not implemented')`