feat(indexing): add catchup task, refactor task handling, and improve reorg error logic

[PJColombo]

New Catchup Task

• Introduces a dedicated task that runs a synchronizer in a separate thread to catch up missing blocks (from the last synced block to the head).
• Ensures catchup can run concurrently while still reindexing incoming head events.

Task Refactor

• Extracts task logic into a dedicated IndexingTask entity responsible for spawning threads and managing synchronizers.
• Improves maintainability by centralizing thread/task handling.

Reorg fix

• It updates the reorg handling logic by throwing an error only when blocks were actually rewound but no common ancestor was found