feat: normalize provider details into oauth_accounts table, auth linking intent, provider array in profile response

pacman-server/migrations/20250917120000_init_users.sql

@@ -0,0 +1,15 @@
+-- users table
+CREATE TABLE IF NOT EXISTS users (
+  id BIGSERIAL PRIMARY KEY,
+  provider TEXT NOT NULL,
+  provider_user_id TEXT NOT NULL,
+  username TEXT NOT NULL,
+  display_name TEXT NULL,
+  email TEXT NULL,
+  avatar_url TEXT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  UNIQUE (provider, provider_user_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_users_provider ON users (provider, provider_user_id);