feat: implement shared-album multi-user photo album

Backend (Go/Gin/pgx): env config, pgx pool with retries, Garage S3 client
(path-style, region garage), embedded golang-migrate migrations, JWT auth with
refresh rotation, albums CRUD, members/roles/invites with no-escalation matrix,
photo upload + async imaging/goexif WebP variants, cursor pagination, presigned
URLs, likes/comments, SSE hub (album + personal), notifications, share links.

Frontend (SvelteKit 2 + Svelte 5 + Tailwind v4): API wrapper with 401 refresh,
auth store, all screens, Uploader/PhotoGrid/Lightbox/Comments, SSE client with
reconnect + REST catch-up, three themes + system via data-theme.

Deploy without Docker: Makefile builds the frontend into backend/static and
embeds it (go:embed) into a single statically linked binary; systemd unit and
nginx SSE example included.

Use gen2brain/webp (-tags nodynamic) instead of cgo kolesa-team/go-webp to keep
CGO_ENABLED=0 static builds; design.md/proposal.md updated accordingly.

backend/internal/sse/serve.go

@@ -0,0 +1,53 @@
+package sse
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+func Serve(c *gin.Context, sub *Subscription, heartbeat time.Duration) {
+	c.Writer.Header().Set("Content-Type", "text/event-stream")
+	c.Writer.Header().Set("Cache-Control", "no-cache")
+	c.Writer.Header().Set("Connection", "keep-alive")
+	c.Writer.Header().Set("X-Accel-Buffering", "no")
+	c.Writer.WriteHeader(http.StatusOK)
+
+	flusher, ok := c.Writer.(http.Flusher)
+	if !ok {
+		c.AbortWithStatus(http.StatusInternalServerError)
+		return
+	}
+	fmt.Fprint(c.Writer, ": connected\n\n")
+	flusher.Flush()
+
+	if heartbeat <= 0 {
+		heartbeat = 25 * time.Second
+	}
+	ticker := time.NewTicker(heartbeat)
+	defer ticker.Stop()
+
+	ctx := c.Request.Context()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case ev, ok := <-sub.Events():
+			if !ok {
+				return
+			}
+			payload, err := json.Marshal(ev.Data)
+			if err != nil {
+				continue
+			}
+			fmt.Fprintf(c.Writer, "event: %s\ndata: %s\n\n", ev.Type, payload)
+			flusher.Flush()
+		case <-ticker.C:
+			fmt.Fprint(c.Writer, ": heartbeat\n\n")
+			flusher.Flush()
+		}
+	}
+}