Paginate C-25 owner-return reply collection (STATE-WP-0094-T03)

GET /messages/ accepts offset with stable created_at,id ordering; C-25 pages
every reply per counterpart, dedups by message id and warns explicitly when
collection is incomplete instead of capping at 200.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 671493@bnt-lap001
Assistant-Session: 60d7b11f-78f9-4731-9367-918511b2762d

api/routers/messages.py

@@ -1,6 +1,6 @@
 from datetime import datetime, timezone
 
-from fastapi import APIRouter, Body, Depends, HTTPException, Request, Response, status
+from fastapi import APIRouter, Body, Depends, HTTPException, Query, Request, Response, status
 from sqlalchemy import and_, or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -137,6 +137,7 @@ async def list_messages(
     from_agent: str | None = None,
     unread_only: bool = False,
     limit: int = 50,
+    offset: int = Query(0, ge=0),
     session: AsyncSession = Depends(get_session),
 ) -> list[MessageRead]:
     now = utcnow()
@@ -170,7 +171,10 @@ async def list_messages(
         values = resolution.slug_values if resolution else (from_agent,)
         query = query.where(AgentMessage.from_agent.in_(values))
     result = await session.execute(
-        query.order_by(AgentMessage.created_at.desc()).limit(limit)
+        # ``id`` breaks created_at ties so offset pages are stable (STATE-WP-0094-T03).
+        query.order_by(AgentMessage.created_at.desc(), AgentMessage.id.desc())
+        .offset(offset)
+        .limit(limit)
     )
     messages = list(result.scalars().all())
     if reader_values is None or reader is None: