Add PushX.Instance for runtime management of named APNS/FCM instances
with start/stop/reconfigure/enable/disable — enabling database-backed
admin panels without app restart.

Fix bare raises in JWT generation and missing :topic validation to
return error tuples instead, add JWT refresh recursion depth limit,
and add :invalid_request/:auth_error response statuses.

Document credential rotation, dynamic instances in README, add
HexDocs module groups, and update CHANGELOG for v0.9.0.