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/migrations/000003_create_publications.down.sql

@@ -0,0 +1,4 @@
+DROP TABLE IF EXISTS publication_fruit_images;
+DROP TABLE IF EXISTS publication_descriptions;
+DROP TABLE IF EXISTS publication_fruits;
+DROP TABLE IF EXISTS publications;

backend/migrations/000003_create_publications.up.sql

@@ -0,0 +1,34 @@
+CREATE TABLE publications (
+  id          SERIAL PRIMARY KEY,
+  title       VARCHAR(255) NOT NULL,
+  author      VARCHAR(255),
+  osdb_pub_id VARCHAR(50)  NOT NULL UNIQUE,
+  image_data  BYTEA,
+  created_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  updated_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE publication_fruits (
+  publication_id INTEGER NOT NULL REFERENCES publications(id) ON DELETE CASCADE,
+  fruit_id       INTEGER NOT NULL REFERENCES fruits(id) ON DELETE CASCADE,
+  PRIMARY KEY (publication_id, fruit_id)
+);
+
+CREATE TABLE publication_descriptions (
+  id             SERIAL PRIMARY KEY,
+  publication_id INTEGER NOT NULL REFERENCES publications(id) ON DELETE CASCADE,
+  fruit_id       INTEGER NOT NULL REFERENCES fruits(id) ON DELETE CASCADE,
+  pdf_data       BYTEA   NOT NULL,
+  created_at     TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  UNIQUE (publication_id, fruit_id)
+);
+
+CREATE TABLE publication_fruit_images (
+  id             SERIAL PRIMARY KEY,
+  publication_id INTEGER      NOT NULL REFERENCES publications(id) ON DELETE CASCADE,
+  fruit_id       INTEGER      NOT NULL REFERENCES fruits(id) ON DELETE CASCADE,
+  filename       VARCHAR(255),
+  data           BYTEA        NOT NULL,
+  image_type     VARCHAR(20)  NOT NULL DEFAULT 'fruit',
+  created_at     TIMESTAMPTZ  NOT NULL DEFAULT NOW()
+);