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/auth/jwt.go

@@ -0,0 +1,82 @@
+package auth
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/hex"
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/google/uuid"
+)
+
+type Claims struct {
+	UserID string `json:"uid"`
+	Email  string `json:"email"`
+	jwt.RegisteredClaims
+}
+
+type TokenManager struct {
+	secret     []byte
+	accessTTL  time.Duration
+	refreshTTL time.Duration
+}
+
+func NewTokenManager(secret string, accessTTL, refreshTTL time.Duration) *TokenManager {
+	return &TokenManager{secret: []byte(secret), accessTTL: accessTTL, refreshTTL: refreshTTL}
+}
+
+func (m *TokenManager) AccessTTL() time.Duration  { return m.accessTTL }
+func (m *TokenManager) RefreshTTL() time.Duration { return m.refreshTTL }
+
+func (m *TokenManager) GenerateAccess(userID, email string) (string, error) {
+	now := time.Now()
+	claims := Claims{
+		UserID: userID,
+		Email:  email,
+		RegisteredClaims: jwt.RegisteredClaims{
+			Subject:   userID,
+			IssuedAt:  jwt.NewNumericDate(now),
+			ExpiresAt: jwt.NewNumericDate(now.Add(m.accessTTL)),
+		},
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString(m.secret)
+}
+
+func (m *TokenManager) ParseAccess(tokenStr string) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenStr, &Claims{}, func(t *jwt.Token) (interface{}, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", t.Header["alg"])
+		}
+		return m.secret, nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	claims, ok := token.Claims.(*Claims)
+	if !ok || !token.Valid {
+		return nil, fmt.Errorf("invalid token")
+	}
+	return claims, nil
+}
+
+func NewRefreshToken() (raw string, hash string, err error) {
+	buf := make([]byte, 32)
+	if _, err = rand.Read(buf); err != nil {
+		return "", "", err
+	}
+	raw = base64.RawURLEncoding.EncodeToString(buf)
+	return raw, HashRefreshToken(raw), nil
+}
+
+func HashRefreshToken(raw string) string {
+	sum := sha256.Sum256([]byte(raw))
+	return hex.EncodeToString(sum[:])
+}
+
+func NewID() string {
+	return uuid.NewString()
+}