feat(backend): add user_devices device identity schema

[Olorunfemi20]

closes #153

Summary

Adds a user_devices table to the backend Drizzle schema to serve as a per-user device identity registry for end-to-end encryption. Each row records a registered device, its platform, and its long-term identity public key. Devices are revoked rather than deleted, so historical sessions remain auditable.

Changes

• New device_platform pgEnum (web / ios / android).
• New user_devices table:
  • id uuid PK, userId uuid FK -> users.id (ON DELETE CASCADE)
  • deviceId, deviceName, identityPublicKey text; platform enum
  • registrationId integer (nullable), lastSeenAt / revokedAt timestamps (nullable)
  • createdAt timestamp default now()
• Unique index on (userId, deviceId) enforcing per-user device uniqueness at the DB level.
• Partial index on userId WHERE revoked_at IS NULL for fast active-device lookups.
• Drizzle relations: users.devices (many) and userDevices.user (one).
• Exported inferred types UserDevice and NewUserDevice.
• Generated migration 0007_user_devices.sql and registered it in the migration journal.

Acceptance criteria

• Migration applies cleanly.
• (userId, deviceId) uniqueness enforced at the database level.
• Revoking a device sets revokedAt without deleting the row.
• Relations and inferred types are exported.

Notes

The table follows the existing schema conventions (snake_case columns, cascade FKs to users, soft-delete via nullable timestamp as used by messages.deletedAt). The migration is hand-authored to match the format and journal pattern of the repository's most recent migrations.

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