add list folder policy api

packages/db/migrations/0180.do.folder_policy.sql

@@ -4,15 +4,18 @@
 
 BEGIN;
 
+CREATE TYPE folder_action AS ENUM ('DELETE', 'ARCHIVE');
+
 CREATE TABLE omnivore.folder_policy (
     id UUID PRIMARY KEY DEFAULT uuid_generate_v1mc(),
     user_id UUID NOT NULL REFERENCES omnivore.user(id) ON DELETE CASCADE,
     folder TEXT NOT NULL, -- folder name in lowercase
-    action TEXT NOT NULL, -- delete or archive
+    action folder_action NOT NULL, -- delete or archive
     after_days INT NOT NULL, -- number of days after which the action should be taken
     minimum_items INT NOT NULL DEFAULT 0, -- minimum number of items to keep in the folder
     created_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    updated_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE (user_id, folder) -- only one policy per folder per user
 );
 
 CREATE TRIGGER update_folder_policy_modtime BEFORE UPDATE ON omnivore.folder_policy FOR EACH ROW EXECUTE PROCEDURE update_updated_at_column();