feat(auth): add user profile, phone login, and account deletion

backend/migrations/000013_user_phone.up.sql

@@ -0,0 +1,6 @@
+-- A second login identifier: phone (E.164), nullable and unique. Email becomes
+-- optional so phone-only accounts are possible; the "at least one identifier"
+-- rule is enforced in the application layer.
+ALTER TABLE users ALTER COLUMN email DROP NOT NULL;
+ALTER TABLE users ADD COLUMN IF NOT EXISTS phone text;
+ALTER TABLE users ADD CONSTRAINT users_phone_key UNIQUE (phone);