feat: add lease and collision visibility to web UI (Closes #433)

Adds read-only /leases and /api/leases routes for issue claim inventory,
reviewer PR lease comments, duplicate PR/branch warnings, and local issue
lock visibility without acquire/release mutations.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

webui/app.py

@@ -14,6 +14,8 @@ from webui.project_registry import find_project, load_registry, registry_to_dict
 from webui.project_views import render_project_detail, render_projects_list
 from webui.prompt_library import find_prompt, library_to_dict
 from webui.prompt_views import render_prompt_detail, render_prompts_page
+from webui.lease_loader import load_lease_snapshot, snapshot_to_dict as lease_snapshot_to_dict
+from webui.lease_views import render_leases_page
 from webui.queue_loader import load_queue_snapshot, snapshot_to_dict
 from webui.queue_views import render_queue_page
 
@@ -141,10 +143,12 @@ async def worktrees(_request: Request) -> HTMLResponse:
 
 
 async def leases(_request: Request) -> HTMLResponse:
-    return _stub_page(
-        "Leases",
-        "Lease visibility will show active issue and reviewer PR leases.",
-    )
+    snapshot = load_lease_snapshot()
+    return HTMLResponse(render_leases_page(snapshot))
+
+
+async def api_leases(_request: Request) -> JSONResponse:
+    return JSONResponse(lease_snapshot_to_dict(load_lease_snapshot()))
 
 
 async def method_not_allowed(request: Request, _exc: Exception) -> Response:
@@ -175,6 +179,7 @@ def create_app() -> Starlette:
             Route("/audit", audit, methods=["GET"]),
             Route("/worktrees", worktrees, methods=["GET"]),
             Route("/leases", leases, methods=["GET"]),
+            Route("/api/leases", api_leases, methods=["GET"]),
         ],
         exception_handlers={405: method_not_allowed},
     )