feat(openresty): add origin error page GET-only option

Allow restricting custom origin error HTML to GET requests so other
methods pass through origin responses. Adds option seed, snapshot field,
edge limit_except/Lua handling, and admin UI switch.

internal/infra/persistence/migrator/goose/postgres/202608060002_add_origin_error_page_get_only.sql

@@ -0,0 +1,8 @@
+-- +goose Up
+INSERT INTO w_system_configs (key, value, type, visibility, description, created_at, updated_at)
+VALUES
+  ('origin_error_page_get_only', 'false', 'business', 0, '源站错误页是否仅对 GET 请求生效（其它方法透传）', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+ON CONFLICT (key) DO NOTHING;
+
+-- +goose Down
+DELETE FROM w_system_configs WHERE key = 'origin_error_page_get_only';

internal/infra/persistence/migrator/goose/sqlite/202608060002_add_origin_error_page_get_only.sql

@@ -0,0 +1,8 @@
+-- +goose Up
+INSERT INTO w_system_configs (key, value, type, visibility, description, created_at, updated_at)
+VALUES
+  ('origin_error_page_get_only', 'false', 'business', 0, '源站错误页是否仅对 GET 请求生效（其它方法透传）', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)
+ON CONFLICT (key) DO NOTHING;
+
+-- +goose Down
+DELETE FROM w_system_configs WHERE key = 'origin_error_page_get_only';