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

backend/migrations/sqlite/000005_user_phone.down.sql

@@ -0,0 +1,21 @@
+-- Email is restored to NOT NULL, so phone-only accounts (email IS NULL) cannot
+-- be represented and are dropped. Down is only safe when no such accounts exist.
+DELETE FROM users WHERE email IS NULL;
+
+CREATE TABLE users_old (
+    id            TEXT PRIMARY KEY,
+    email         TEXT NOT NULL UNIQUE,
+    password_hash TEXT NOT NULL,
+    name          TEXT NOT NULL,
+    avatar_url    TEXT,
+    preferences   TEXT NOT NULL DEFAULT '{}',
+    created_at    TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+
+INSERT INTO users_old (id, email, password_hash, name, avatar_url, preferences, created_at)
+SELECT id, email, password_hash, name, avatar_url, preferences, created_at
+FROM users WHERE email IS NOT NULL;
+
+DROP TABLE users;
+
+ALTER TABLE users_old RENAME TO users;