[NOTICKET] refactor: mint message_id as UUID to mirror analyses_messages id shape

Change the Python-minted assistant turn id from msg_<12hex> to a canonical
UUID string (str(uuid.uuid4())) in both mint sites (v2 chat + help), so the
value is format-compatible with Go's analyses_messages.id (UUID). Still
Python-minted and independent; only the shape now matches. No schema change
(message_traceability.message_id stays String). Stale msg_<12hex> comments
in models.py updated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/api/v1/help.py

@@ -51,8 +51,9 @@ async def help_stream(request: HelpRequest, db: AsyncSession = Depends(get_db)):
       2. chunk    — text fragments of the guidance
       3. done     — `{"message_id": "..."}` for the observability lookup
     """
-    # Server-authoritative turn id — never accepted from the caller (keys /observability).
-    message_id = f"msg_{uuid.uuid4().hex[:12]}"
+    # Server-authoritative turn id — never accepted from the caller (keys traceability).
+    # Canonical UUID string, matching Go's `analyses_messages.id` shape (mirrors v2 chat).
+    message_id = str(uuid.uuid4())
     try:
         history = await load_history(db, request.analysis_id, limit=10)
 

src/api/v2/chat.py

@@ -70,8 +70,11 @@ async def _save_empty_chat_trace(analysis_id: str, user_id: str, message_id: str
 
 def _mint_message_id() -> str:
     """Mint the assistant turn id. Server-authoritative — never accepted from the caller
-    (it keys the future /observability lookup). Returned on `done`; open-Q #1 resolved."""
-    return f"msg_{uuid.uuid4().hex[:12]}"
+    (it keys the GET /api/v1/traceability lookup). Returned on `done`; open-Q #1 resolved.
+
+    A canonical UUID string, matching Go's `analyses_messages.id` shape, so the value stays
+    format-compatible if we later swap to the real message-row id (still Python-minted now)."""
+    return str(uuid.uuid4())
 
 
 class ChatRequest(BaseModel):

src/db/postgres/models.py

@@ -265,7 +265,7 @@ class MessageTraceabilityRow(Base):
     (src\\traceability\\schemas.py:TraceabilityPayload) serialized via
     `model_dump(mode="json", by_alias=True)`; the read path rehydrates with
     `TraceabilityPayload.model_validate(...)`. One row per assistant `message_id`
-    (the Python-minted turn id "msg_<12hex>"), written before the `done` SSE event
+    (the Python-minted turn id, a UUID string), written before the `done` SSE event
     and served by `GET /api/v1/traceability`.
 
     OWNERSHIP / HANDOFF (KM-691): **Python-owned for now**, the same pattern as
@@ -278,7 +278,7 @@ class MessageTraceabilityRow(Base):
     """
     __tablename__ = "message_traceability"
 
-    message_id = Column(String, primary_key=True)  # Python-minted turn id ("msg_<12hex>")
+    message_id = Column(String, primary_key=True)  # Python-minted turn id (UUID string)
     analysis_id = Column(UUID(as_uuid=False), nullable=False, index=True)  # analysis session id
     user_id = Column(String, nullable=False)
     intent = Column(String, nullable=False)