Add OAuth backend scaffold

backend/schema.sql

@@ -0,0 +1,33 @@
+create extension if not exists pgcrypto;
+
+create table if not exists users (
+    id uuid primary key default gen_random_uuid(),
+    email text unique,
+    display_name text not null default '',
+    avatar_url text,
+    password_hash text,
+    created_at timestamptz not null default now(),
+    updated_at timestamptz not null default now()
+);
+
+create table if not exists identities (
+    id uuid primary key default gen_random_uuid(),
+    user_id uuid not null references users(id) on delete cascade,
+    provider text not null check (provider in ('apple', 'google', 'email')),
+    provider_subject text not null,
+    email text,
+    created_at timestamptz not null default now(),
+    unique(provider, provider_subject)
+);
+
+create table if not exists premium_entitlements (
+    user_id uuid primary key references users(id) on delete cascade,
+    source text not null default 'app_store',
+    product_id text not null,
+    active boolean not null default false,
+    expires_at timestamptz,
+    updated_at timestamptz not null default now()
+);
+
+create index if not exists identities_user_id_idx on identities(user_id);
+create index if not exists premium_entitlements_active_idx on premium_entitlements(active);