feat: inject build version/commit into scraper and UI at docker build time

- Go scraper: Version/Commit vars in main.go, injected via -ldflags; Server struct + New() updated; GET /health and new GET /api/version expose them
- UI Dockerfile: ARG BUILD_VERSION/BUILD_COMMIT → ENV PUBLIC_BUILD_VERSION/PUBLIC_BUILD_COMMIT for SvelteKit
- Footer: shows version+short commit when not 'dev' (text-zinc-800, subtle)
- docker-compose: args blocks for scraper and ui build sections pass $GIT_TAG/$GIT_COMMIT

ui/Dockerfile

@@ -5,6 +5,15 @@ COPY package.json package-lock.json ./
 RUN npm ci
 
 COPY . .
+
+# Build-time version info — injected by docker-compose or CI via --build-arg.
+ARG BUILD_VERSION=dev
+ARG BUILD_COMMIT=unknown
+
+# Expose as PUBLIC_ env vars so SvelteKit's $env/dynamic/public can read them.
+ENV PUBLIC_BUILD_VERSION=$BUILD_VERSION
+ENV PUBLIC_BUILD_COMMIT=$BUILD_COMMIT
+
 RUN npm run build
 
 # ── Runtime image ──────────────────────────────────────────────────────────────

ui/src/routes/+layout.svelte

@@ -5,6 +5,7 @@
 	import type { Snippet } from 'svelte';
 	import type { LayoutData } from './$types';
 	import { audioStore } from '$lib/audio.svelte';
+	import { env } from '$env/dynamic/public';
 
 	let { children, data }: { children: Snippet; data: LayoutData } = $props();
 
@@ -395,6 +396,9 @@
 				<a href="/privacy" class="hover:text-zinc-500 transition-colors">Privacy</a>
 				<a href="/dmca" class="hover:text-zinc-500 transition-colors">DMCA</a>
 				<span>&copy; {new Date().getFullYear()} libnovel</span>
+				{#if env.PUBLIC_BUILD_VERSION && env.PUBLIC_BUILD_VERSION !== 'dev'}
+					<span class="text-zinc-800">{env.PUBLIC_BUILD_VERSION}+{env.PUBLIC_BUILD_COMMIT?.slice(0, 7)}</span>
+				{/if}
 			</div>
 		</div>
 	</footer>