docs: Fix message IDs in API documentation to show database UUIDs

Update 06-get-messages.md to show accurate message ID format. Message IDs
are database-generated UUIDs, not client-generated IDs from useChat.

Changes:
- Replace example IDs "msg-1", "msg-2" with realistic UUIDs
- Clarify in Response Fields that IDs are "database-generated UUID"
- Add explanatory note about message ID behavior:
* Client-provided IDs are ignored by server
* Server generates new database UUIDs for all messages
* Database UUIDs must be used for branching/quoting operations

This aligns documentation with actual API behavior as validated by
message-id-consistency.api.spec.ts tests.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/backend-doc/06-get-messages.md

@@ -24,7 +24,7 @@ curl -X GET http://localhost:3000/api/conversations/403db363-ee32-4ec9-aae0-2017
 {
   "messages": [
     {
-      "id": "msg-1",
+      "id": "1ef1e965-2bcd-4494-aeb8-eaf07dee2870",
       "conversationId": "403db363-ee32-4ec9-aae0-2017af936d52",
       "role": "user",
       "speaker": "student",
@@ -32,7 +32,7 @@ curl -X GET http://localhost:3000/api/conversations/403db363-ee32-4ec9-aae0-2017
       "timestamp": "2025-01-27T10:30:00Z"
     },
     {
-      "id": "msg-2",
+      "id": "2af2f076-3def-4605-beb9-fbc18f1ef981",
       "conversationId": "403db363-ee32-4ec9-aae0-2017af936d52",
       "role": "assistant",
       "speaker": "student",
@@ -40,7 +40,7 @@ curl -X GET http://localhost:3000/api/conversations/403db363-ee32-4ec9-aae0-2017
       "timestamp": "2025-01-27T10:30:15Z"
     },
     {
-      "id": "msg-3",
+      "id": "3bf3a187-4efg-5716-cfca-acd29b2fa092",
       "conversationId": "403db363-ee32-4ec9-aae0-2017af936d52",
       "role": "user",
       "speaker": "coach",
@@ -48,7 +48,7 @@ curl -X GET http://localhost:3000/api/conversations/403db363-ee32-4ec9-aae0-2017
       "timestamp": "2025-01-27T10:31:00Z"
     },
     {
-      "id": "msg-4",
+      "id": "4ca4b298-5fab-6827-dadb-bde3ac3fb1a3",
       "conversationId": "403db363-ee32-4ec9-aae0-2017af936d52",
       "role": "assistant",
       "speaker": "coach",
@@ -64,7 +64,7 @@ curl -X GET http://localhost:3000/api/conversations/403db363-ee32-4ec9-aae0-2017
 
 Each message includes:
 
-- `id` (string) - Unique message identifier
+- `id` (string) - Unique message identifier (database-generated UUID)
 - `conversationId` (string) - Parent conversation ID
 - `role` (string) - Message role
   - `"user"` - Message from teacher
@@ -79,6 +79,8 @@ Each message includes:
 - `quotedText` (string, optional) - Text being replied to
 - `quotedMessageId` (string, optional) - ID of quoted message
 
+**Note on Message IDs**: Message IDs are database-generated UUIDs in the format `xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx`. Client-provided IDs (from useChat) are ignored by the server. Always use the database UUID returned in the API response for operations like branching or quoting messages.
+
 ## Message Types
 
 ### User Message (to Student)