fix: remove redundant .unique() on anonymous_devices.deviceId

[jakebromberg]

Summary

• The anonymous_devices table had both .unique() on the deviceId column and an explicit uniqueIndex('anonymous_devices_device_id_key') in the table's third argument, creating two redundant unique indexes on device_id.
• Removed the inline .unique() and kept the explicit uniqueIndex for naming control.
• Added a schema assertion test that verifies exactly one uniqueness constraint exists on device_id.

Test plan

• New unit test tests/unit/database/schema.anonymous-devices.test.ts fails before fix (count = 2) and passes after (count = 1)
• Existing unit tests pass (npm run test:unit)
• No migration generated — this is a schema-definition-only change; the database already has the named index

Made with Cursor

Note: This fix is consolidated in #199 (coordinated schema migration batch). Consider closing in favor of #199.