feat: DELETE /unregister endpoint

[gastonm5]

Summary

• Add DELETE /unregister/:vault_id/:party_name endpoint to allow devices to remove their push notification registration by vault ID and local party ID
• Add UnregisterDeviceByParty storage method that hard-deletes the device record matching the composite unique index (vault_id, party_name)
• Update README with endpoint documentation

Test plan

• Call DELETE /unregister/<vault_id>/<party_name> with a registered device and verify the record is removed from the database
• Call DELETE /unregister/<vault_id>/<party_name> with a non-existent combo and verify a 500 response
• Verify that POST /register still works after unregistering (re-registration)

[johnnyluo]

LGTM