feat(health): expose app version in GET /health

Surface the deployed app version (app.version, = pyproject 0.1.0) in the
health response for observability. Reuses the FastAPI instance attribute —
no new settings field, no runtime pyproject parsing.

Refs #717

backend/app/main.py

@@ -172,4 +172,8 @@ app.include_router(ping.router, prefix="/api/v1", tags=["ping"])
 
 @app.get("/health")
 async def health() -> dict[str, str]:
-    return {"status": "ok", "environment": settings.environment}
+    return {
+        "status": "ok",
+        "environment": settings.environment,
+        "version": app.version,
+    }