feat(backend): initialize Go module and implement domain layer

- Add go.mod (module tanabata/backend, Go 1.21) with uuid dependency
- Implement internal/domain: File, Tag, TagRule, Category, Pool, PoolFile,
  User, Session, Permission, ObjectType, AuditEntry + all pagination types
- Add domain error sentinels (ErrNotFound, ErrForbidden, etc.)
- Add context helpers WithUser/UserFromContext for JWT propagation
- Fix migration: remove redundant DEFAULT on exif jsonb column

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

backend/internal/domain/user.go

@@ -0,0 +1,39 @@
+package domain
+
+import "time"
+
+// User is an application user.
+type User struct {
+	ID         int16
+	Name       string
+	Password   string // bcrypt hash; only populated when needed for auth
+	IsAdmin    bool
+	CanCreate  bool
+	IsBlocked  bool
+}
+
+// Session is an active user session.
+type Session struct {
+	ID           int
+	TokenHash    string
+	UserID       int16
+	UserAgent    string
+	StartedAt    time.Time
+	ExpiresAt    *time.Time
+	LastActivity time.Time
+	IsCurrent    bool // true when this session matches the caller's token
+}
+
+// OffsetPage is a generic offset-based page of users.
+type UserPage struct {
+	Items  []User
+	Total  int
+	Offset int
+	Limit  int
+}
+
+// SessionList is a list of sessions with a total count.
+type SessionList struct {
+	Items []Session
+	Total int
+}