FastAPI skeleton: health, OpenAPI, core wiring (task 042dfc5c)

Add api/ with a FastAPI app that imports the bundled NestQuest core
(Feature 15) WITHOUT importing homeassistant: api/nestquest_core.py
bootstraps the integration directory onto sys.path and imports core as
a top-level package, so custom_components/nestquest/__init__.py (which
imports HA) never executes. This transitional coupling is encapsulated
in one documented helper to revisit in Feature 18.

The app reads NESTQUEST_DB_PATH (env var, documented temp default),
opens+migrates the SQLite database in a FastAPI lifespan, and serves
GET /health (200, status ok + resolved db_path) and the OpenAPI doc at
/docs and /openapi.json. Tests start the app with httpx against the
ASGI app and assert all three; a subprocess test proves the app
imports core without importing homeassistant or registering
custom_components.

Add fastapi, uvicorn, httpx to the dev dependency group; uv lock.

Suite green at 1581 (1576 baseline + 5 new API tests).

pyproject.toml

@@ -8,10 +8,13 @@ dependencies = []
 
 [dependency-groups]
 dev = [
+    "fastapi>=0.115",
+    "httpx>=0.28",
     "jinja2>=3",
     "pytest>=8.0.0",
     "pytest-asyncio>=0.24",
     "pyyaml>=6",
+    "uvicorn>=0.30",
     "voluptuous>=0.14",
 ]