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/audit.go

@@ -0,0 +1,46 @@
+package domain
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// ActionType is a reference entity for auditable user actions.
+type ActionType struct {
+	ID   int16
+	Name string
+}
+
+// AuditEntry is a single audit log record.
+type AuditEntry struct {
+	ID           int64
+	UserID       int16
+	UserName     string // denormalized
+	Action       string // action type name, e.g. "file_create"
+	ObjectType   *string
+	ObjectID     *uuid.UUID
+	Details      json.RawMessage
+	PerformedAt  time.Time
+}
+
+// AuditPage is an offset-based page of audit log entries.
+type AuditPage struct {
+	Items  []AuditEntry
+	Total  int
+	Offset int
+	Limit  int
+}
+
+// AuditFilter holds filter parameters for querying the audit log.
+type AuditFilter struct {
+	UserID     *int16
+	Action     string
+	ObjectType string
+	ObjectID   *uuid.UUID
+	From       *time.Time
+	To         *time.Time
+	Offset     int
+	Limit      int
+}