feat(di): Add DatabaseModule with Hilt providers for Room DB and DAOs (#19)

- DatabaseModule: @Module + @InstallIn(SingletonComponent) with @Singleton-scoped
  Room.databaseBuilder provider and four @Provides methods for ItemDao,
  CategoryDao, LocationDao and SettingsDao
- DatabaseModuleTest: smoke-test verifies all four DAO providers return
  non-null objects using an in-memory Room database

app/src/androidTest/java/de/krisenvorrat/app/di/DatabaseModuleTest.kt

@@ -0,0 +1,46 @@
+package de.krisenvorrat.app.di
+
+import android.content.Context
+import androidx.room.Room
+import androidx.test.core.app.ApplicationProvider
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import de.krisenvorrat.app.data.db.KrisenvorratDatabase
+import org.junit.After
+import org.junit.Assert.assertNotNull
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@RunWith(AndroidJUnit4::class)
+internal class DatabaseModuleTest {
+
+    private lateinit var db: KrisenvorratDatabase
+
+    @After
+    fun tearDown() {
+        if (::db.isInitialized) {
+            db.close()
+        }
+    }
+
+    @Test
+    fun test_provideAllDaos_withInMemoryDb_allDaosAreNonNull() {
+        // Given
+        val context = ApplicationProvider.getApplicationContext<Context>()
+        val module = DatabaseModule
+
+        // When
+        db = Room.inMemoryDatabaseBuilder(context, KrisenvorratDatabase::class.java)
+            .allowMainThreadQueries()
+            .build()
+        val itemDao = module.provideItemDao(db)
+        val categoryDao = module.provideCategoryDao(db)
+        val locationDao = module.provideLocationDao(db)
+        val settingsDao = module.provideSettingsDao(db)
+
+        // Then
+        assertNotNull(itemDao)
+        assertNotNull(categoryDao)
+        assertNotNull(locationDao)
+        assertNotNull(settingsDao)
+    }
+}

app/src/main/java/de/krisenvorrat/app/di/DatabaseModule.kt

@@ -0,0 +1,37 @@
+package de.krisenvorrat.app.di
+
+import android.content.Context
+import androidx.room.Room
+import dagger.Module
+import dagger.Provides
+import dagger.hilt.InstallIn
+import dagger.hilt.android.qualifiers.ApplicationContext
+import dagger.hilt.components.SingletonComponent
+import de.krisenvorrat.app.data.db.KrisenvorratDatabase
+import de.krisenvorrat.app.data.db.dao.CategoryDao
+import de.krisenvorrat.app.data.db.dao.ItemDao
+import de.krisenvorrat.app.data.db.dao.LocationDao
+import de.krisenvorrat.app.data.db.dao.SettingsDao
+import javax.inject.Singleton
+
+@Module
+@InstallIn(SingletonComponent::class)
+internal object DatabaseModule {
+
+    @Provides
+    @Singleton
+    fun provideDatabase(@ApplicationContext context: Context): KrisenvorratDatabase =
+        Room.databaseBuilder(context, KrisenvorratDatabase::class.java, "krisenvorrat.db").build()
+
+    @Provides
+    fun provideItemDao(db: KrisenvorratDatabase): ItemDao = db.itemDao()
+
+    @Provides
+    fun provideCategoryDao(db: KrisenvorratDatabase): CategoryDao = db.categoryDao()
+
+    @Provides
+    fun provideLocationDao(db: KrisenvorratDatabase): LocationDao = db.locationDao()
+
+    @Provides
+    fun provideSettingsDao(db: KrisenvorratDatabase): SettingsDao = db.settingsDao()
+}