Slice 6: notifications per §15

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api_branches.py

@@ -901,6 +901,35 @@ def make_router(
         )
         return {"ok": True, "message_id": message_id}
 
+    @router.post("/api/rfcs/{slug}/branches/{branch}/chat-seen")
+    async def advance_chat_seen(slug: str, branch: str, body: dict, request: Request) -> dict[str, Any]:
+        """§15.7 chat-seen cursor advance.
+
+        Body: `{"last_seen_message_id": <int>}`. Upserts branch_chat_seen
+        and runs the §15.7 reconciler — every unread notification scoped
+        to this (slug, branch) on or before the new cursor is marked read.
+        """
+        viewer = auth.require_user(request)
+        _require_rfc_with_repo(slug)
+        if not _can_read_branch(slug, branch, viewer):
+            raise HTTPException(403, "Branch is private")
+        last_seen = int(body.get("last_seen_message_id") or 0) or None
+        db.conn().execute(
+            """
+            INSERT INTO branch_chat_seen (user_id, rfc_slug, branch_name, last_seen_message_id, seen_at)
+            VALUES (?, ?, ?, ?, datetime('now'))
+            ON CONFLICT(user_id, rfc_slug, branch_name) DO UPDATE SET
+                last_seen_message_id = excluded.last_seen_message_id,
+                seen_at = excluded.seen_at
+            """,
+            (viewer.user_id, slug, branch, last_seen),
+        )
+        from . import notify
+        reconciled = notify.reconcile_seen_advance(
+            user_id=viewer.user_id, rfc_slug=slug, branch_name=branch,
+        )
+        return {"ok": True, "reconciled": reconciled}
+
     @router.post("/api/rfcs/{slug}/branches/{branch}/threads/{thread_id}/resolve")
     async def resolve_thread(slug: str, branch: str, thread_id: int, request: Request) -> dict[str, Any]:
         viewer = auth.require_contributor(request)