feat(migrate): configure embedded Goose providers (#14)

Add embedded SQL and provider-local Go migrations with slog logging and
Postgres session locking, returning Goose's Provider directly.

Cover timestamp and sequential versions, failure recovery, and concurrent
startup with SQLite and Postgres tests. Reuse page's Postgres CI service.
Document authoring conventions and the Go template compatibility gate.

Closes #7
Refs #9

migrate/testdata/numeric/00010_event.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+INSERT INTO migration_events (version) VALUES (10);
+
+-- +goose Down
+DELETE FROM migration_events WHERE version = 10;