feat(backup): add uploadBackupSnapshot and deleteBackupSnapshot to FirebaseStorageDataSource

app/src/main/java/app/closer/data/remote/FirebaseStorageDataSource.kt

@@ -68,6 +68,37 @@ class FirebaseStorageDataSource @Inject constructor(
                 .addOnFailureListener { cont.resumeWithException(it) }
         }
 
+    /**
+     * Uploads an already-encrypted conversation-backup snapshot under the uploader's OWN path and
+     * returns the tokenized download URL (recorded in the couple-gated backup manifest so the partner
+     * can fetch it). The bytes are couple-key ciphertext, so Storage holds nothing readable. Living
+     * under users/{uid}/ means the existing account-delete cleanup covers backups too.
+     */
+    suspend fun uploadBackupSnapshot(uid: String, snapshotId: String, encryptedBytes: ByteArray): String =
+        suspendCancellableCoroutine { cont ->
+            if (!isOnline()) {
+                cont.resumeWithException(IOException("No internet connection"))
+                return@suspendCancellableCoroutine
+            }
+            val ref = storage.reference.child("users/$uid/backups/$snapshotId")
+            val metadata = StorageMetadata.Builder()
+                .setContentType("application/octet-stream")
+                .build()
+            ref.putBytes(encryptedBytes, metadata)
+                .continueWithTask { ref.downloadUrl }
+                .addOnSuccessListener { cont.resume(it.toString()) }
+                .addOnFailureListener { cont.resumeWithException(it) }
+        }
+
+    /** Best-effort delete of a previously-uploaded backup snapshot (post-compaction cleanup; succeeds
+     *  only for the uploader's own path, so a cross-partner stale blob is left for that owner/cleanup). */
+    suspend fun deleteBackupSnapshot(uid: String, snapshotId: String): Unit =
+        suspendCancellableCoroutine { cont ->
+            storage.reference.child("users/$uid/backups/$snapshotId").delete()
+                .addOnSuccessListener { cont.resume(Unit) }
+                .addOnFailureListener { cont.resume(Unit) } // best-effort; a leftover blob is harmless ciphertext
+        }
+
     /**
      * Downloads the raw (still-encrypted) bytes for a media message over HTTP using the tokenized
      * download URL, so the partner can read the author's object (the URL token authorizes it,