increase cardinality of platform accounts

apps/auth/migrations/0007_platform_accounts.sql

@@ -0,0 +1,35 @@
+-- Let one account be linked to MORE THAN ONE platform identity, so a player with a
+-- PC and a headset gets a cached login on both. The account blob's single
+-- `platformId`/`platform` pair could only hold one, so logging in on the second
+-- device meant a password every time.
+--
+-- Links move into their own table, which becomes the one source of truth for both
+-- halves of a cached login (the picker and the `cached_login` grant). The blob fields
+-- stay as the account's *primary* identity — the first one linked — for the account
+-- DTO and the refresh grant's claims; nothing authorizes off them any more. Kept in
+-- sync with PLATFORM_SCHEMA_DDL in src/platform-db.ts.
+
+CREATE TABLE IF NOT EXISTS platform_account (
+  account_id INTEGER NOT NULL,
+  platform INTEGER NOT NULL,
+  platform_id TEXT NOT NULL,
+  linked_at TEXT NOT NULL,
+  PRIMARY KEY (platform, platform_id, account_id)
+  );
+CREATE INDEX IF NOT EXISTS idx_platform_account_account ON platform_account (account_id);
+CREATE INDEX IF NOT EXISTS idx_platform_account_platform_id ON platform_account (platform_id);
+
+-- Backfill every identity already bound to an account. `platform` is COALESCEd to 0
+-- because nothing ever defaulted that field: an account can carry a platformId with no
+-- platform recorded, and back when Steam was the only verifiable platform an unset one
+-- *was* Steam. Without the COALESCE those accounts would lose their cached login at
+-- deploy. Mirrored as PLATFORM_BACKFILL_SQL in src/platform-db.ts, which is what the
+-- tests run.
+INSERT OR IGNORE INTO platform_account (account_id, platform, platform_id, linked_at)
+SELECT
+  account_id,
+  COALESCE(json_extract(data, '$.platform'), 0),
+  platform_id,
+  COALESCE(json_extract(data, '$.createdAt'), '1970-01-01T00:00:00Z')
+FROM account
+WHERE platform_id IS NOT NULL AND platform_id <> '';