Adopt golib/pg for migrations and pool construction

encapi's schema was a cumulative IF NOT EXISTS blob re-executed inline on every
start: it grows forever, records nothing, and cannot express a change that is
not a fresh CREATE. golib owns that mechanism now, so encapi keeps the SQL and
drops the runner.

- Move the DDL verbatim into migrations/0001_init.sql, embedded via
  migrations.FS. It stays IF NOT EXISTS-guarded, so the first start against the
  live database re-runs it as a no-op and only lands the schema_migrations row.
- Build the pool with pg.NewMigrated under the advisory lock named
  encapi-migrations, and delete the inline migrate(). database.New now takes a
  context and a logger; main.go hands it the signal context so a start blocked
  on the migration lock still dies on SIGTERM.
- Render the DSN with pg.DSN. The env var contract is untouched — the fields are
  still resolved by internal/config, because encapi defaults DBUSER and DBNAME
  to "encapi" where pg.DSNFromEnv treats both as required.
- Guard the set: embedded files must match migrations/, every CREATE must be
  idempotent, the derived lock key is pinned, and a container test proves the
  adoption path over a database that predates schema_migrations.
- Plumb GOPRIVATE=git.unkin.net for the first cross-repo Go dependency:
  exported by the Makefile, set in the Dockerfile and the woodpecker Go steps,
  documented in the README.

migrations/0001_init.sql

@@ -0,0 +1,23 @@
+CREATE TABLE IF NOT EXISTS statuses (
+	name        TEXT PRIMARY KEY,
+	description TEXT NOT NULL DEFAULT '',
+	created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+	updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS roles (
+	name           TEXT PRIMARY KEY,
+	description    TEXT NOT NULL DEFAULT '',
+	default_params JSONB NOT NULL DEFAULT '{}',
+	created_at     TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+	updated_at     TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS nodes (
+	certname    TEXT PRIMARY KEY,
+	role        TEXT NOT NULL REFERENCES roles(name) ON UPDATE CASCADE,
+	environment TEXT NOT NULL REFERENCES statuses(name) ON UPDATE CASCADE,
+	params      JSONB NOT NULL DEFAULT '{}',
+	created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+	updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);