Backup: versioned JSON serializer, SigV4 + S3 client, JCE crypto, daily worker

Milestone 5. One BackupManager entry point so export, import and
auto-backup share the same serializer and crypto by construction.
Explicit DTOs decouple the backup contract from the Room schema.
JCE baseline (PBKDF2-HMAC-SHA256 600k → AES-256-GCM) behind the
BackupCrypto interface with format auto-detection on decrypt; age
lands behind the same interface in milestone 6. SigV4 signer verified
against AWS's published documentation example signature. Settings in
SharedPreferences encrypted under a non-exportable AndroidKeyStore key
— replaces the now fully deprecated androidx security-crypto with the
same construction (~70 lines, zero deps); threat-model comments state
plainly that the stored auto-backup passphrase only protects against
bucket compromise. WorkManager daily periodic job, inexact by design.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/test/java/no/naiv/meddetsamme/backup/JceBackupCryptoTest.kt

@@ -0,0 +1,54 @@
+package no.naiv.meddetsamme.backup
+
+import javax.crypto.AEADBadTagException
+import org.junit.Assert.assertArrayEquals
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertThrows
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class JceBackupCryptoTest {
+
+    private val plaintext = """{"version":1,"medications":[]}""".toByteArray()
+    private val passphrase = "korrekt hest batteri stift".toCharArray()
+
+    @Test
+    fun `round trip with correct passphrase`() {
+        val ct = JceBackupCrypto.encrypt(plaintext, passphrase)
+        assertArrayEquals(plaintext, JceBackupCrypto.decrypt(ct, passphrase))
+    }
+
+    @Test
+    fun `wrong passphrase fails authentication, not garbage output`() {
+        val ct = JceBackupCrypto.encrypt(plaintext, passphrase)
+        assertThrows(AEADBadTagException::class.java) {
+            JceBackupCrypto.decrypt(ct, "feil passord".toCharArray())
+        }
+    }
+
+    @Test
+    fun `tampered ciphertext fails authentication`() {
+        val ct = JceBackupCrypto.encrypt(plaintext, passphrase)
+        ct[ct.size - 5] = (ct[ct.size - 5].toInt() xor 1).toByte()
+        assertThrows(AEADBadTagException::class.java) {
+            JceBackupCrypto.decrypt(ct, passphrase)
+        }
+    }
+
+    @Test
+    fun `format detection by magic`() {
+        val ct = JceBackupCrypto.encrypt(plaintext, passphrase)
+        assertTrue(JceBackupCrypto.matches(ct))
+        assertFalse(JceBackupCrypto.matches(plaintext))
+        assertEquals(JceBackupCrypto, BackupCrypto.detect(ct))
+        assertEquals(null, BackupCrypto.detect(plaintext))
+    }
+
+    @Test
+    fun `fresh salt and IV every time`() {
+        val a = JceBackupCrypto.encrypt(plaintext, passphrase)
+        val b = JceBackupCrypto.encrypt(plaintext, passphrase)
+        assertFalse(a.contentEquals(b)) // deterministic encryption would leak equality
+    }
+}