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

pacman-server/migrations/20250917121000_oauth_accounts.sql

@@ -0,0 +1,18 @@
+-- OAuth accounts linked to a single user
+CREATE TABLE IF NOT EXISTS oauth_accounts (
+  id BIGSERIAL PRIMARY KEY,
+  user_id BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  provider TEXT NOT NULL,
+  provider_user_id TEXT NOT NULL,
+  email TEXT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  UNIQUE (provider, provider_user_id)
+);
+
+-- Ensure we can look up by email efficiently
+CREATE INDEX IF NOT EXISTS idx_oauth_accounts_email ON oauth_accounts (email);
+
+-- Optional: ensure users email uniqueness if desired; keep NULLs allowed
+ALTER TABLE users
+  ADD CONSTRAINT users_email_unique UNIQUE (email);