feat: manage fruits — full CRUD with images and synonyms (story #02)

Backend: domain structs, FruitRepository interface + pg implementation,
9 Echo v4 handlers (list/get/create/update/delete, image sub-resources),
migration 000002 (fruit_type ENUM, fruits, fruit_synonyms, fruit_images),
route-scoped BodyLimit("5M") for uploads, http.DetectContentType for serving.

Frontend: typed fetch API layer, Pinia setup-style fruitStore, FruitList
(paginated), FruitCreate, and FruitDetail (edit + synonyms editor + image
gallery). 25 backend unit tests + integration test; 18 frontend tests.

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

backend/migrations/000002_create_fruits.down.sql

@@ -0,0 +1,4 @@
+DROP TABLE IF EXISTS fruit_images;
+DROP TABLE IF EXISTS fruit_synonyms;
+DROP TABLE IF EXISTS fruits;
+DROP TYPE  IF EXISTS fruit_type;

backend/migrations/000002_create_fruits.up.sql

@@ -0,0 +1,45 @@
+CREATE TYPE fruit_type AS ENUM (
+  'Apfelsorten',
+  'Birnensorten',
+  'Quittensorten',
+  'Aprikosen',
+  'Pfirsiche',
+  'Mirabellen',
+  'Renekloden',
+  'Pflaumen',
+  'Zwetschen',
+  'Sauerkirschen',
+  'Süßkirschen',
+  'Brombeeren',
+  'Erdbeeren',
+  'Himbeeren',
+  'Johannisbeeren',
+  'Stachelbeeren',
+  'Wein'
+);
+
+CREATE TABLE fruits (
+  id          SERIAL       PRIMARY KEY,
+  name        VARCHAR(255) NOT NULL,
+  osdb_number VARCHAR(50)  NOT NULL UNIQUE,
+  comment     TEXT,
+  fruit_type  fruit_type   NOT NULL,
+  created_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  updated_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE fruit_synonyms (
+  id       SERIAL       PRIMARY KEY,
+  fruit_id INTEGER      NOT NULL REFERENCES fruits(id) ON DELETE CASCADE,
+  synonym  VARCHAR(255) NOT NULL
+);
+
+CREATE TABLE fruit_images (
+  id         SERIAL       PRIMARY KEY,
+  fruit_id   INTEGER      NOT NULL REFERENCES fruits(id) ON DELETE CASCADE,
+  filename   VARCHAR(255),
+  data       BYTEA        NOT NULL,
+  image_type VARCHAR(20)  NOT NULL CHECK (image_type IN ('fruit', 'flower', 'tree')),
+  title      VARCHAR(255),
+  created_at TIMESTAMPTZ  NOT NULL DEFAULT NOW()
+);