api/v1: add code-first Huma implementation of the v1 API

Reimplement the v1 API as a code-first Huma service in hscontrol/api/v1,
a thin adapter over the state layer. Huma emits the OpenAPI 3.1 document
(openapi/v1/headscale.yaml) from the Go operation and type definitions;
cmd/gen-openapi writes it and the 3.0.3 downgrade the client is generated
from. Responses reproduce the protojson wire contract (string-encoded
64-bit IDs, all fields emitted, RFC3339/null timestamps); errors map to
the correct HTTP status (404/400/409, 500 for server faults) via mapError.

Serve it on the chi router at /api/v1 behind the existing API-key
middleware, and point /swagger at the emitted spec.

.gitignore

@@ -46,6 +46,9 @@ result
 
 integration_test/etc/config.dump.yaml
 
+# OpenAPI spec is served live from the code and emitted on demand, not committed
+/openapi/v1/headscale.yaml
+
 # MkDocs
 .cache
 /site