refactor!: disk-first zero-copy architecture with file-to-file cwebp conversion

BREAKING CHANGE: Complete refactoring of the conversion pipeline.

- Replace in-memory Page/PageContainer with disk-based PageFile struct
- Extract archives to temp directory instead of loading into memory
- Use cwebp InputFile/OutputFile for zero-copy file-to-file conversion
- Use cwebp -crop for splitting (no Go-side image decode needed)
- Check if already converted before extracting (fast pre-check)
- Remove oliamb/cutter dependency (replaced by cwebp -crop)
- Remove page_container.go (no longer needed)
- Add comprehensive tests with improved coverage

Co-authored-by: Belphemur <197810+Belphemur@users.noreply.github.com>

internal/manga/page.go

@@ -1,86 +1,16 @@
 package manga
 
-import (
-	"bytes"
-	"fmt"
-	"io"
-	"os"
-
-	"github.com/rs/zerolog/log"
-)
-
-type Page struct {
-	// Index of the page in the chapter.
-	Index uint16 `json:"index" jsonschema:"description=Index of the page in the chapter."`
-	// Extension of the page image.
-	Extension string `json:"extension" jsonschema:"description=Extension of the page image."`
-	// Size of the page in bytes
-	Size uint64 `json:"-"`
-	// Contents of the page. Nil when the page contents have been staged to
-	// disk (see TempFilePath) to bound memory usage.
-	Contents *bytes.Buffer `json:"-"`
-	// TempFilePath, when non-empty, points to a file on disk (in a staging
-	// temp folder) holding the page contents instead of keeping them fully
-	// in memory. Use Open() to transparently read the page contents
-	// regardless of where they are stored.
-	TempFilePath string `json:"-"`
-	// IsSplitted tell us if the page was cropped to multiple pieces
-	IsSplitted bool `json:"is_cropped" jsonschema:"description=Was this page cropped."`
-	// SplitPartIndex represent the index of the crop if the page was cropped
-	SplitPartIndex uint16 `json:"crop_part_index" jsonschema:"description=Index of the crop if the image was cropped."`
-}
-
-// Open returns a reader for the page contents, transparently handling
-// whether the contents are held in memory (Contents) or staged on disk
-// (TempFilePath). The caller is responsible for closing the returned
-// reader.
-func (page *Page) Open() (io.ReadCloser, error) {
-	if page.TempFilePath != "" {
-		file, err := os.Open(page.TempFilePath)
-		if err != nil {
-			return nil, fmt.Errorf("failed to open staged page contents: %w", err)
-		}
-		return file, nil
-	}
-	if page.Contents == nil {
-		return nil, fmt.Errorf("page has no contents: neither TempFilePath nor Contents is set")
-	}
-	return io.NopCloser(bytes.NewReader(page.Contents.Bytes())), nil
-}
-
-// Stage writes the given content to a file in tempDir instead of keeping it
-// in memory, updating Extension, Size and TempFilePath accordingly and
-// clearing Contents. This is used after converting a page so that only the
-// pages currently being processed are held in memory, bounding memory usage
-// for chapters with many/large pages.
-func (page *Page) Stage(tempDir string, content *bytes.Buffer, extension string) error {
-	file, err := os.CreateTemp(tempDir, "page-*.tmp")
-	if err != nil {
-		return fmt.Errorf("failed to create staging file: %w", err)
-	}
-	fileName := file.Name()
-
-	written, writeErr := file.Write(content.Bytes())
-	closeErr := file.Close()
-	if writeErr == nil && closeErr == nil && written == content.Len() {
-		page.TempFilePath = fileName
-		page.Extension = extension
-		page.Size = uint64(written)
-		page.Contents = nil
-		return nil
-	}
-
-	// Something went wrong: remove the incomplete/partial staging file
-	// rather than leaving corrupted data behind on disk.
-	if removeErr := os.Remove(fileName); removeErr != nil {
-		log.Warn().Str("file", fileName).Err(removeErr).Msg("Failed to remove incomplete staging file")
-	}
-
-	if writeErr != nil {
-		return fmt.Errorf("failed to write staging file: %w", writeErr)
-	}
-	if closeErr != nil {
-		return fmt.Errorf("failed to close staging file: %w", closeErr)
-	}
-	return fmt.Errorf("short write to staging file: wrote %d of %d bytes", written, content.Len())
+// PageFile represents a single page image stored on disk.
+// No image data is held in memory — only metadata and a file path.
+type PageFile struct {
+	// Index of the page in the chapter (original ordering).
+	Index uint16
+	// Extension of the page image file (e.g., ".webp", ".jpg").
+	Extension string
+	// FilePath is the absolute path to the image file on disk.
+	FilePath string
+	// IsSplitted indicates whether this page was split from a larger image.
+	IsSplitted bool
+	// SplitPartIndex is the part index when the page was split.
+	SplitPartIndex uint16
 }