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

backend/migrations/000013_user_phone.down.sql

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+ALTER TABLE users DROP CONSTRAINT IF EXISTS users_phone_key;
+ALTER TABLE users DROP COLUMN IF EXISTS phone;
+ALTER TABLE users ALTER COLUMN email SET NOT NULL;

backend/migrations/000013_user_phone.up.sql

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+-- 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);

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

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+-- 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;

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

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+-- 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;