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/file.go

@@ -0,0 +1,54 @@
+package domain
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// MIMEType holds MIME whitelist data.
+type MIMEType struct {
+	ID        int16
+	Name      string
+	Extension string
+}
+
+// File represents a managed file record.
+type File struct {
+	ID              uuid.UUID
+	OriginalName    *string
+	MIMEType        string // denormalized from core.mime_types
+	MIMEExtension   string // denormalized from core.mime_types
+	ContentDatetime time.Time
+	Notes           *string
+	Metadata        json.RawMessage
+	EXIF            json.RawMessage
+	PHash           *int64
+	CreatorID       int16
+	CreatorName     string // denormalized from core.users
+	IsPublic        bool
+	IsDeleted       bool
+	CreatedAt       time.Time // extracted from UUID v7
+	Tags            []Tag     // loaded with the file
+}
+
+// FileListParams holds all parameters for listing/filtering files.
+type FileListParams struct {
+	Filter    string
+	Sort      string
+	Order     string
+	Cursor    string
+	Anchor    *uuid.UUID
+	Direction string // "forward" or "backward"
+	Limit     int
+	Trash     bool
+	Search    string
+}
+
+// FilePage is the result of a cursor-based file listing.
+type FilePage struct {
+	Items      []File
+	NextCursor *string
+	PrevCursor *string
+}