Wire the frontend into the deploy stack and CI

Compose gains a frontend service; Caddy now routes / to frontend:3000 (keeping /api/* and /health* on the backend). CI builds and pushes a frontend image alongside the backend. Verified end-to-end on the deploy target: / serves the app, /api and /health still resolve through Caddy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

deploy/Caddyfile

@@ -15,11 +15,8 @@
 		reverse_proxy backend:8000
 	}
 
-	# Frontend (Next.js) — not yet deployed in Phase 0. Uncomment when it lands.
-	# handle {
-	#	reverse_proxy frontend:3000
-	# }
+	# Everything else -> the Next.js frontend.
 	handle {
-		respond "Provenance — Phase 0. Backend health at /health; frontend not yet deployed." 200
+		reverse_proxy frontend:3000
 	}
 }