feat: SHA-256 checksums (#15) and S3 storage backend (#16)

#15 — Backup Integrity Verification:
- Compute SHA-256 hash after archive creation, store in checksum column
- "Verify Integrity" toolbar button re-computes and compares
- Hash logged in backup step log

#16 — S3-Compatible Storage:
- S3Uploader with AWS Signature V4 (no SDK dependency, pure cURL)
- Single PUT for files <= 100 MB, multipart upload for larger files
- Multipart: 10 MB parts, abort on failure, XML completion
- Works with AWS S3, Wasabi, Backblaze B2, MinIO (custom endpoints)
- Profile form fields: endpoint, region, access key, secret key, bucket, path
- showon conditional visibility (only shown when remote_storage=s3)
- Akeeba importer maps S3 credentials from Akeeba profiles
- Added to BackupEngine createUploader() factory

Authored-by: Moko Consulting
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/packages/com_mokobackup/sql/install.mysql.sql

@@ -10,7 +10,7 @@ CREATE TABLE IF NOT EXISTS `#__mokobackup_profiles` (
 	`exclude_dirs`      TEXT NOT NULL COMMENT 'Newline-separated directory paths to exclude',
 	`exclude_files`     TEXT NOT NULL COMMENT 'Newline-separated filename patterns to exclude',
 	`exclude_tables`    TEXT NOT NULL COMMENT 'Newline-separated table names to exclude',
-	`remote_storage`    VARCHAR(20) NOT NULL DEFAULT 'none' COMMENT 'none, ftp, google_drive',
+	`remote_storage`    VARCHAR(20) NOT NULL DEFAULT 'none' COMMENT 'none, ftp, google_drive, s3',
 	`ftp_host`          VARCHAR(255) NOT NULL DEFAULT '',
 	`ftp_port`          INT(5) UNSIGNED NOT NULL DEFAULT 21,
 	`ftp_username`      VARCHAR(255) NOT NULL DEFAULT '',
@@ -22,6 +22,12 @@ CREATE TABLE IF NOT EXISTS `#__mokobackup_profiles` (
 	`gdrive_client_secret` VARCHAR(255) NOT NULL DEFAULT '',
 	`gdrive_refresh_token` VARCHAR(512) NOT NULL DEFAULT '',
 	`gdrive_folder_id`     VARCHAR(255) NOT NULL DEFAULT '',
+	`s3_endpoint`          VARCHAR(512) NOT NULL DEFAULT '' COMMENT 'S3 endpoint URL (blank = AWS default)',
+	`s3_region`            VARCHAR(50) NOT NULL DEFAULT 'us-east-1',
+	`s3_access_key`        VARCHAR(255) NOT NULL DEFAULT '',
+	`s3_secret_key`        VARCHAR(255) NOT NULL DEFAULT '',
+	`s3_bucket`            VARCHAR(255) NOT NULL DEFAULT '',
+	`s3_path`              VARCHAR(512) NOT NULL DEFAULT '/backups',
 	`remote_keep_local`    TINYINT(1) NOT NULL DEFAULT 1 COMMENT 'Keep local copy after upload',
 	`include_kickstart`    TINYINT(1) NOT NULL DEFAULT 0 COMMENT 'Include standalone restore.php in archive',
 	`notify_email`         VARCHAR(512) NOT NULL DEFAULT '' COMMENT 'Comma-separated notification emails',
@@ -54,6 +60,7 @@ CREATE TABLE IF NOT EXISTS `#__mokobackup_records` (
 	`backupend`       DATETIME NOT NULL DEFAULT '0000-00-00 00:00:00',
 	`filesexist`      TINYINT(1) NOT NULL DEFAULT 1,
 	`remote_filename` VARCHAR(512) NOT NULL DEFAULT '',
+	`checksum`        VARCHAR(64) NOT NULL DEFAULT '' COMMENT 'SHA-256 hash of archive',
 	`log`             MEDIUMTEXT NOT NULL COMMENT 'Step-by-step backup log',
 	PRIMARY KEY (`id`),
 	KEY `idx_profile` (`profile_id`),