Phase 0: fixes from review (torn files, pending blobs, exact sequences, UTF-8, storage checks)

- detect files changing during reading or between resumable steps and
  re-read them (size/mtime/ctime/inode), spike F overwrites mid-run
- drop pending blob ids when a pack or index upload fails
- dump and restore PostgreSQL sequence state exactly, bound parameters,
  validated catalog names; spike C checks all sequences exactly
- keep invalid UTF-8 byte-exact (tagged base64)
- refuse large tables without a primary key
- StorageBackend: check write/flush/close results, validate base path
- user_migration import/export: root paths, intermediate folders,
  partial writes, read errors
- spikes: never touch an existing account, own fixtures, 512M note

lib/Repository/PackWriter.php

@@ -59,9 +59,17 @@ final class PackWriter {
 		fwrite($this->buffer, $header . pack('N', strlen($header)));
 		rewind($this->buffer);
 		$path = 'packs/' . substr($packId, 0, 2) . '/' . $packId;
-		$this->backend->put($path, $this->buffer); // closes the buffer
-		unset($this->buffer);
-		$this->index->addPack($packId, $this->entries);
+		try {
+			$this->backend->put($path, $this->buffer); // closes the buffer
+			unset($this->buffer);
+			$this->index->addPack($packId, $this->entries);
+		} catch (\Throwable $e) {
+			// The blobs of this pack are not durable: forget them, so a retry stores them again.
+			$this->index->dropPending(array_column($this->entries, 'id'));
+			unset($this->buffer);
+			$this->reset();
+			throw $e;
+		}
 		$this->reset();
 	}