feat(webhook): derive work_item_id from Plane sequence_id (M-6)

src/plane_sync.py

@@ -71,6 +71,24 @@ STAGE_TO_STATE = {
 }
 
 
+def fetch_issue_sequence_id(issue_id: str, project_id: str) -> int | None:
+    """M-6: GET the Plane issue by UUID and return its sequence_id (the
+    authoritative per-project number), or None if unavailable.
+
+    Returns None on network error, non-2xx, or a missing field - never raises,
+    so the webhook handler can fall back to DB increment and stay autonomous.
+    """
+    url = f"{PLANE_BASE}/workspaces/{WORKSPACE}/projects/{project_id}/issues/{issue_id}/"
+    try:
+        resp = httpx.get(url, headers=PLANE_HEADERS, timeout=10)
+        resp.raise_for_status()
+        seq = resp.json().get("sequence_id")
+        return int(seq) if seq is not None else None
+    except Exception as e:
+        logger.warning(f"fetch_issue_sequence_id failed for {issue_id}: {e}")
+        return None
+
+
 def find_issue_id(work_item_id: str, project_id: str = None) -> str | None:
     """Find Plane issue UUID by work_item_id (e.g. 'ET-002')."""
     project_id = _resolve_project_id(work_item_id, project_id)