feat(indexer): add bounded write queue with fetch backpressure

[Enchanterme]

Add BoundedEventQueue between event fetch and Postgres writes. When the buffer is full, fetch pauses and drains a write batch first so bursts of on-chain events cannot pile up unbounded memory. Configurable via INDEXER_MAX_QUEUE_SIZE and INDEXER_WRITE_BATCH_SIZE.

Description

Please include a summary of the change and which issue is fixed.

Fixes # (issue)

Closes #264
Closes #263
Closes #262
Closes #265

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist:

• My code follows the coding conventions
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[drips-wave]

@Enchanterme Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits