feat: implement gallery feed with social features and SSE

- Cursor-based feed endpoint using v_feed view with hashtag filtering
- Like toggle (INSERT ON CONFLICT), comments CRUD
- Feed delta endpoint for SSE-driven incremental updates
- SSE client with Page Visibility API (pause/reconnect)
- Responsive photo/video grid with infinite scroll
- Hashtag filter chips, lightbox modal with comments
- Media file serving via tower-http ServeDir

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/sse.ts

@@ -0,0 +1,86 @@
+import { getToken } from './auth';
+
+type EventHandler = (data: string) => void;
+
+let eventSource: EventSource | null = null;
+let lastEventTime: string | null = null;
+const handlers: Map<string, EventHandler[]> = new Map();
+
+export function onSseEvent(eventType: string, handler: EventHandler): () => void {
+	if (!handlers.has(eventType)) {
+		handlers.set(eventType, []);
+	}
+	handlers.get(eventType)!.push(handler);
+
+	// Return unsubscribe function
+	return () => {
+		const list = handlers.get(eventType);
+		if (list) {
+			const idx = list.indexOf(handler);
+			if (idx >= 0) list.splice(idx, 1);
+		}
+	};
+}
+
+export function connectSse(): void {
+	const token = getToken();
+	if (!token || eventSource) return;
+
+	// EventSource doesn't support custom headers, so pass token as query param
+	// The backend will need to accept this — or we use a polyfill / fetch-based SSE
+	// For simplicity, use native EventSource with token in URL
+	eventSource = new EventSource(`/api/v1/stream?token=${encodeURIComponent(token)}`);
+
+	eventSource.onopen = () => {
+		lastEventTime = new Date().toISOString();
+	};
+
+	eventSource.addEventListener('new-upload', (e) => dispatch('new-upload', e.data));
+	eventSource.addEventListener('upload-processed', (e) => dispatch('upload-processed', e.data));
+	eventSource.addEventListener('like-update', (e) => dispatch('like-update', e.data));
+	eventSource.addEventListener('new-comment', (e) => dispatch('new-comment', e.data));
+	eventSource.addEventListener('export-available', (e) => dispatch('export-available', e.data));
+
+	eventSource.onerror = () => {
+		// EventSource auto-reconnects, but we track the time for delta-fetch
+		disconnectSse();
+		// Reconnect after a short delay
+		setTimeout(connectSse, 3000);
+	};
+}
+
+export function disconnectSse(): void {
+	if (eventSource) {
+		eventSource.close();
+		eventSource = null;
+	}
+}
+
+export function getLastEventTime(): string | null {
+	return lastEventTime;
+}
+
+export function setLastEventTime(time: string): void {
+	lastEventTime = time;
+}
+
+function dispatch(eventType: string, data: string): void {
+	lastEventTime = new Date().toISOString();
+	const list = handlers.get(eventType);
+	if (list) {
+		for (const handler of list) {
+			handler(data);
+		}
+	}
+}
+
+// Page Visibility API integration
+if (typeof document !== 'undefined') {
+	document.addEventListener('visibilitychange', () => {
+		if (document.hidden) {
+			disconnectSse();
+		} else {
+			connectSse();
+		}
+	});
+}