feat(backend): implement port interfaces (repository and storage)

Define all repository interfaces in port/repository.go:
FileRepo, TagRepo, TagRuleRepo, CategoryRepo, PoolRepo, UserRepo,
SessionRepo, ACLRepo, AuditRepo, MimeRepo, and Transactor.
Add OffsetParams and PoolFileListParams as shared parameter structs.

Define FileStorage interface in port/storage.go with Save, Read,
Delete, Thumbnail, and Preview methods.

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

backend/internal/port/storage.go

@@ -0,0 +1,31 @@
+package port
+
+import (
+	"context"
+	"io"
+
+	"github.com/google/uuid"
+)
+
+// FileStorage abstracts disk (or object-store) operations for file content,
+// thumbnails, and previews.
+type FileStorage interface {
+	// Save writes the reader's content to storage and returns the number of
+	// bytes written. ext is the file extension without a leading dot (e.g. "jpg").
+	Save(ctx context.Context, id uuid.UUID, ext string, r io.Reader) (int64, error)
+
+	// Read opens the file content for reading. The caller must close the returned
+	// ReadCloser.
+	Read(ctx context.Context, id uuid.UUID, ext string) (io.ReadCloser, error)
+
+	// Delete removes the file content from storage.
+	Delete(ctx context.Context, id uuid.UUID, ext string) error
+
+	// Thumbnail opens the pre-generated thumbnail (JPEG). Returns ErrNotFound
+	// if the thumbnail has not been generated yet.
+	Thumbnail(ctx context.Context, id uuid.UUID) (io.ReadCloser, error)
+
+	// Preview opens the pre-generated preview image (JPEG). Returns ErrNotFound
+	// if the preview has not been generated yet.
+	Preview(ctx context.Context, id uuid.UUID) (io.ReadCloser, error)
+}