fix(backup): incremental backups against S3 + jsonb stats parsing

Three fixes uncovered while bringing the backup-s3 integration suite to
12/12 against MinIO + Postgres:

- backupService.getDatabaseBackupInfo: pg's jsonb driver auto-parses
  `statistics` / `table_checksums` to objects; the old JSON.parse() then
  threw "[object Object]" is not valid JSON and the manifest dropped
  database info silently. Accept both string and object inputs.

- backupService.runBackup: incremental path called
  backupManifest.loadManifest() with an s3:// URI directly, which falls
  through to fs.readFile() and ENOENTs — every "incremental" backup
  silently downgraded to a full one. Added loadManifestFromAnywhere()
  helper that downloads s3:// to a tmp file before delegating.

- backupManifest.generateIncrementalManifest: attached the `incremental`
  section AFTER generateManifest() had already stamped
  verification.total_checksum, so every incremental manifest failed
  validateManifest() on read-back. Recompute the checksum after.

Test side: updated assertions to the current manifest shape
(`incremental.changes.modified_files_count`), Number()-coerce bigint
columns from pg, and gate the logger mock on UNMOCK_LOGGER for
diagnosing similar silent-failure modes in the future.

backend/__tests__/integration/backup-s3.test.js

@@ -80,10 +80,12 @@ describe('S3 Backup Integration Tests', () => {
     await setupTestData();
 
     // Mock logger to reduce noise
-    logger.info = jest.fn();
-    logger.debug = jest.fn();
-    logger.warn = jest.fn();
-    logger.error = jest.fn();
+    if (process.env.UNMOCK_LOGGER !== 'true') {
+      logger.info = jest.fn();
+      logger.debug = jest.fn();
+      logger.warn = jest.fn();
+      logger.error = jest.fn();
+    }
   });
 
   afterAll(async () => {
@@ -281,13 +283,16 @@ describe('S3 Backup Integration Tests', () => {
         .first();
 
       expect(secondRun.id).not.toBe(firstRun.id);
-      expect(secondRun.files_backed_up).toBe(1); // Only modified file
+      expect(Number(secondRun.files_backed_up)).toBe(1); // Only modified file
 
-      // Check manifest indicates incremental
+      // Check manifest indicates incremental. The current manifest schema
+      // groups counts under `incremental.changes.*` (added/modified/deleted/
+      // unchanged + size_difference) — see backupManifest.generateIncrementalManifest.
       if (secondRun.manifest_path) {
         const manifest = await backupService.getBackupManifest(secondRun.id);
         expect(manifest.manifest.incremental).toBeDefined();
-        expect(manifest.manifest.incremental.modified_files_count).toBe(1);
+        expect(manifest.manifest.incremental.changes).toBeDefined();
+        expect(manifest.manifest.incremental.changes.modified_files_count).toBe(1);
       }
     });