diff --git a/backend/internal/ports/errors.go b/backend/internal/ports/errors.go
new file mode 100644
index 0000000..1cc6dfa
--- /dev/null
+++ b/backend/internal/ports/errors.go
@@ -0,0 +1,24 @@
+package ports
+
+import (
+	"errors"
+
+	"booklib/internal/store"
+)
+
+// Re-export sentinel errors so handlers can use errors.Is via ports.
+var (
+	ErrLastAdmin       = store.ErrLastAdmin
+	ErrUniqueViolation = store.ErrUniqueViolation
+)
+
+// IsUniqueViolation consolidates the pg 23505 check into a single predicate.
+func IsUniqueViolation(err error) bool { return store.IsUniqueViolation(err) }
+
+// Upload sentinel errors.
+var (
+	ErrTooLarge     = errors.New("file too large")
+	ErrIncomplete   = errors.New("upload incomplete")
+	ErrSizeMismatch = errors.New("size mismatch")
+	ErrNotFound     = errors.New("not found")
+)
diff --git a/backend/internal/ports/ports.go b/backend/internal/ports/ports.go
new file mode 100644
index 0000000..ec4f698
--- /dev/null
+++ b/backend/internal/ports/ports.go
@@ -0,0 +1,99 @@
+// Package ports defines consumer-side interfaces for the backend services.
+// Handlers depend on these interfaces, not on concrete implementations.
+// Main.go wires concrete types (*store.Store, *redispkg.R, etc.) that satisfy them.
+//
+// Value types (User, Book, etc.) live in internal/store and are referenced here.
+package ports
+
+import (
+	"context"
+	"io"
+	"time"
+
+	"booklib/internal/store"
+)
+
+// ---------- Store interfaces ----------
+
+type UserStore interface {
+	CountUsers(ctx context.Context) (int, error)
+	CreateUser(ctx context.Context, username, hash, role string) (int64, error)
+	GetUserByName(ctx context.Context, username string) (store.User, error)
+	GetUserByID(ctx context.Context, id int64) (store.User, error)
+	ListUsers(ctx context.Context) ([]store.User, error)
+	DeleteUser(ctx context.Context, id int64) error
+}
+
+type LibraryStore interface {
+	CreateLibrary(ctx context.Context, name, root string) (int64, error)
+	ListLibraries(ctx context.Context) ([]store.Library, error)
+	GetLibrary(ctx context.Context, id int64) (store.Library, error)
+}
+
+type BookStore interface {
+	InsertBook(ctx context.Context, libID int64, path, title, format string, size, modTS int64, pageCount int) (int64, error)
+	GetBook(ctx context.Context, id int64) (store.Book, error)
+	ListBookMeta(ctx context.Context, libID int64) (map[string]store.BookMeta, error)
+	UpdateBookFile(ctx context.Context, id, size, modTS int64, pageCount int) error
+	DeleteBookByPath(ctx context.Context, libID int64, path string) error
+	DeleteBook(ctx context.Context, id int64) error
+	SetBookState(ctx context.Context, id int64, state, msg string) error
+	ListBooks(ctx context.Context, libID int64, q, prefix string, userID int64) ([]store.BookView, error)
+	BookHashes(ctx context.Context) (map[int64][2]int64, error)
+}
+
+type ProgressStore interface {
+	UpsertProgress(ctx context.Context, userID, libID int64, bookPath string, locator []byte, percent float64) error
+	GetProgress(ctx context.Context, userID, libID int64, bookPath string) (store.Progress, error)
+	ListProgress(ctx context.Context, userID int64) ([]store.Progress, error)
+}
+
+type BookmarkStore interface {
+	InsertBookmark(ctx context.Context, userID, libID int64, bookPath string, locator []byte, percent float64, note string) (int64, error)
+	ListBookmarks(ctx context.Context, userID, libID int64, bookPath string) ([]store.Bookmark, error)
+	UpdateBookmarkNote(ctx context.Context, userID, id int64, note string) (bool, error)
+	DeleteBookmark(ctx context.Context, userID, id int64) (bool, error)
+}
+
+// ---------- Redis interfaces ----------
+
+type PageCache interface {
+	Get(ctx context.Context, key string) (string, bool)
+	Set(ctx context.Context, key, val string, ttl time.Duration)
+}
+
+type RateLimiter interface {
+	IncrWindow(ctx context.Context, key string, ttl time.Duration) int
+}
+
+type ScanLocker interface {
+	ScanLock(ctx context.Context, key string, ttl time.Duration) (func(), bool)
+}
+
+// ---------- Service interfaces ----------
+
+type Scanner interface {
+	ScanLibraryByID(ctx context.Context, id int64)
+}
+
+type UploadSessions interface {
+	Init(ctx context.Context, libID int64, name string, size, chunkSize int64) (string, error)
+	Status(ctx context.Context, uploadID string) ([]int64, error)
+	PutPart(ctx context.Context, uploadID string, index int64, body io.Reader, maxSize int64) error
+	Complete(ctx context.Context, uploadID string, root string) (string, error)
+	Sweep(ctx context.Context) error
+	UniquePath(root, name string) (string, error)
+}
+
+type Media interface {
+	EnsureCover(ctx context.Context, bookID int64, format string, size, modTS int64, root, rel string) error
+	ChaptersOf(idx []string) []Chapter
+	PageIndex(ctx context.Context, bookID int64, size, modTS int64, root, rel string) ([]string, error)
+	CacheBuster(size, modTS int64) string
+}
+
+// Chapter represents a CBZ chapter derived from the archive's folder structure.
+type Chapter struct {
+	Title string `json:"title"`
+	Start int    `json:"start"`
+}
diff --git a/backend/internal/store/store.go b/backend/internal/store/store.go
index 70e5121..ed2ff85 100644
--- a/backend/internal/store/store.go
+++ b/backend/internal/store/store.go
@@ -13,6 +13,9 @@ import (
 // ErrLastAdmin is returned by DeleteUser when attempting to delete the last admin.
 var ErrLastAdmin = errors.New("cannot delete the last admin")
 
+// ErrUniqueViolation is a sentinel for PostgreSQL unique constraint violations.
+var ErrUniqueViolation = errors.New("unique violation")
+
 // IsUniqueViolation reports whether err is a PostgreSQL unique constraint violation (23505).
 func IsUniqueViolation(err error) bool {
 	var pgErr *pgconn.PgError