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

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

@@ -0,0 +1,20 @@
+-- SQLite cannot drop a NOT NULL constraint via ALTER, so rebuild the users
+-- table: email becomes nullable, phone is added as a unique second identifier,
+-- and existing rows are carried across (phone stays NULL).
+CREATE TABLE users_new (
+    id            TEXT PRIMARY KEY,
+    email         TEXT UNIQUE,
+    phone         TEXT 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_new (id, email, phone, password_hash, name, avatar_url, preferences, created_at)
+SELECT id, email, NULL, password_hash, name, avatar_url, preferences, created_at FROM users;
+
+DROP TABLE users;
+
+ALTER TABLE users_new RENAME TO users;