test(db): expect schema v20 in encryption round-trip after v19->v20 bump

MIGRATION_19_20 (issue #187) bumped the Room cache schema to version 20,
but DatabaseEncryptionTest.schemaVersionIsCarriedOntoTheEncryptedFile still
asserted the plaintext -> encrypted conversion carried version 19, so it
failed across all E2E levels after the rebase onto main.

DatabaseEncryption.migrate() carries PRAGMA user_version dynamically
(userVersion = source.version -> target.version = userVersion), and a fresh
Room open now stamps 20, so v20 genuinely survives the conversion. Update the
expected constant to 20; the assertion's intent (the version survives the
round-trip) is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/androidTest/kotlin/org/libremail/data/local/DatabaseEncryptionTest.kt

@@ -147,7 +147,7 @@ class DatabaseEncryptionTest {
         } finally {
             encrypted.close()
         }
-        assertEquals("Room's schema version must survive the plaintext -> encrypted conversion", 19, version)
+        assertEquals("Room's schema version must survive the plaintext -> encrypted conversion", 20, version)
     }
 
     private fun openPlaintext(): LibreMailDatabase =