feat: publication management with cover images, PDFs, and fruit images (story #04)

Full CRUD for publications; link fruits to publications; upload per-fruit
PDF descriptions and fruit images stored as BYTEA; fruit detail page shows
publication descriptions and images. ImageOverlayDrawer for cover thumbnail
full-size view.

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

backend/internal/domain/publication.go

@@ -0,0 +1,47 @@
+package domain
+
+import "time"
+
+type Publication struct {
+	ID         int       `json:"id"`
+	Title      string    `json:"title"`
+	Author     *string   `json:"author"`
+	OSDBPubID  string    `json:"osdb_pub_id"`
+	ImageURL   *string   `json:"image_url,omitempty"`
+	CreatedAt  time.Time `json:"created_at"`
+	UpdatedAt  time.Time `json:"updated_at"`
+}
+
+type PublicationWriteDTO struct {
+	Title     string  `json:"title"`
+	Author    *string `json:"author"`
+	OSDBPubID string  `json:"osdb_pub_id"`
+}
+
+type PublicationFruit struct {
+	FruitID    int    `json:"fruit_id"`
+	Name       string `json:"name"`
+	OSDBNumber string `json:"osdb_number"`
+}
+
+type PublicationDescription struct {
+	ID               int       `json:"id"`
+	PublicationID    int       `json:"publication_id"`
+	FruitID          int       `json:"fruit_id"`
+	FruitName        string    `json:"fruit_name"`
+	PublicationTitle string    `json:"publication_title"`
+	URL              string    `json:"url"`
+	CreatedAt        time.Time `json:"created_at"`
+}
+
+type PublicationFruitImage struct {
+	ID                int       `json:"id"`
+	PublicationID     int       `json:"publication_id"`
+	PublicationTitle  string    `json:"publication_title"`
+	PublicationAuthor *string   `json:"publication_author"`
+	FruitID           int       `json:"fruit_id"`
+	Filename          *string   `json:"filename"`
+	ImageType         string    `json:"image_type"`
+	URL               string    `json:"url"`
+	CreatedAt         time.Time `json:"created_at"`
+}