fix: add build-system to pyproject.toml for correct package discovery

Without a [build-system] table, uv sync in CI failed to discover nested
subpackages like spec2event.adapters.build, causing ModuleNotFoundError
in test collection.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: raphael-solace

apps/api/pyproject.toml

@@ -1,3 +1,10 @@
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/spec2event"]
+
 [project]
 name = "spec2event-api"
 version = "0.1.0"