feat: 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/acl.go

@@ -0,0 +1,19 @@
+package domain
+
+import "github.com/google/uuid"
+
+// ObjectType is a reference entity (file, tag, category, pool).
+type ObjectType struct {
+	ID   int16
+	Name string
+}
+
+// Permission represents a per-object access entry for a user.
+type Permission struct {
+	UserID        int16
+	UserName      string // denormalized
+	ObjectTypeID  int16
+	ObjectID      uuid.UUID
+	CanView       bool
+	CanEdit       bool
+}