feat(#17): Room-Entities & LocalDateConverter

- Add CategoryEntity, LocationEntity, SettingsEntity, ItemEntity
- ItemEntity: FK to Category+Location with CASCADE, indices on FK columns
- LocalDateConverter: LocalDate? <-> String? (ISO-8601) via @TypeConverter
- Add LocalDateConverterTest: 4 unit tests (null/non-null round-trip)

app/src/main/java/de/krisenvorrat/app/data/db/LocalDateConverter.kt

@@ -0,0 +1,13 @@
+package de.krisenvorrat.app.data.db
+
+import androidx.room.TypeConverter
+import java.time.LocalDate
+
+internal class LocalDateConverter {
+
+    @TypeConverter
+    fun fromLocalDate(date: LocalDate?): String? = date?.toString()
+
+    @TypeConverter
+    fun toLocalDate(value: String?): LocalDate? = value?.let { LocalDate.parse(it) }
+}

app/src/main/java/de/krisenvorrat/app/data/db/entity/CategoryEntity.kt

@@ -0,0 +1,11 @@
+package de.krisenvorrat.app.data.db.entity
+
+import androidx.room.ColumnInfo
+import androidx.room.Entity
+import androidx.room.PrimaryKey
+
+@Entity(tableName = "categories")
+internal data class CategoryEntity(
+    @PrimaryKey(autoGenerate = true) val id: Int = 0,
+    @ColumnInfo(name = "name") val name: String
+)

app/src/main/java/de/krisenvorrat/app/data/db/entity/ItemEntity.kt

@@ -0,0 +1,41 @@
+package de.krisenvorrat.app.data.db.entity
+
+import androidx.room.ColumnInfo
+import androidx.room.Entity
+import androidx.room.ForeignKey
+import androidx.room.Index
+import androidx.room.PrimaryKey
+import java.time.LocalDate
+
+@Entity(
+    tableName = "items",
+    foreignKeys = [
+        ForeignKey(
+            entity = CategoryEntity::class,
+            parentColumns = ["id"],
+            childColumns = ["category_id"],
+            onDelete = ForeignKey.CASCADE
+        ),
+        ForeignKey(
+            entity = LocationEntity::class,
+            parentColumns = ["id"],
+            childColumns = ["location_id"],
+            onDelete = ForeignKey.CASCADE
+        )
+    ],
+    indices = [Index("category_id"), Index("location_id")]
+)
+internal data class ItemEntity(
+    @PrimaryKey val id: String,
+    @ColumnInfo(name = "name") val name: String,
+    @ColumnInfo(name = "category_id") val categoryId: Int,
+    @ColumnInfo(name = "quantity") val quantity: Double,
+    @ColumnInfo(name = "unit") val unit: String,
+    @ColumnInfo(name = "unit_price") val unitPrice: Double,
+    @ColumnInfo(name = "kcal_per_100g") val kcalPer100g: Int?,
+    @ColumnInfo(name = "expiry_date") val expiryDate: LocalDate?,
+    @ColumnInfo(name = "location_id") val locationId: Int,
+    @ColumnInfo(name = "min_stock") val minStock: Double,
+    @ColumnInfo(name = "notes") val notes: String,
+    @ColumnInfo(name = "last_updated") val lastUpdated: Long
+)

app/src/main/java/de/krisenvorrat/app/data/db/entity/LocationEntity.kt

@@ -0,0 +1,11 @@
+package de.krisenvorrat.app.data.db.entity
+
+import androidx.room.ColumnInfo
+import androidx.room.Entity
+import androidx.room.PrimaryKey
+
+@Entity(tableName = "locations")
+internal data class LocationEntity(
+    @PrimaryKey(autoGenerate = true) val id: Int = 0,
+    @ColumnInfo(name = "name") val name: String
+)

app/src/main/java/de/krisenvorrat/app/data/db/entity/SettingsEntity.kt

@@ -0,0 +1,11 @@
+package de.krisenvorrat.app.data.db.entity
+
+import androidx.room.ColumnInfo
+import androidx.room.Entity
+import androidx.room.PrimaryKey
+
+@Entity(tableName = "settings")
+internal data class SettingsEntity(
+    @PrimaryKey val key: String,
+    @ColumnInfo(name = "value") val value: String
+)

app/src/test/java/de/krisenvorrat/app/data/db/LocalDateConverterTest.kt

@@ -0,0 +1,59 @@
+package de.krisenvorrat.app.data.db
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+import java.time.LocalDate
+
+class LocalDateConverterTest {
+
+    private val converter = LocalDateConverter()
+
+    @Test
+    fun test_fromLocalDate_withDate_returnsIsoString() {
+        // Given
+        val date = LocalDate.of(2025, 12, 31)
+
+        // When
+        val result = converter.fromLocalDate(date)
+
+        // Then
+        assertEquals("2025-12-31", result)
+    }
+
+    @Test
+    fun test_fromLocalDate_withNull_returnsNull() {
+        // Given
+        val date: LocalDate? = null
+
+        // When
+        val result = converter.fromLocalDate(date)
+
+        // Then
+        assertNull(result)
+    }
+
+    @Test
+    fun test_toLocalDate_withIsoString_returnsDate() {
+        // Given
+        val isoString = "2025-12-31"
+
+        // When
+        val result = converter.toLocalDate(isoString)
+
+        // Then
+        assertEquals(LocalDate.of(2025, 12, 31), result)
+    }
+
+    @Test
+    fun test_toLocalDate_withNull_returnsNull() {
+        // Given
+        val isoString: String? = null
+
+        // When
+        val result = converter.toLocalDate(isoString)
+
+        // Then
+        assertNull(result)
+    }
+}