enhancement(connectors): mid-sync checkpoint state so large syncs resume on crash #8146

Problem

AbstractConnector.sync() updates last_sync_at only on successful completion. If a large sync crashes mid-way (OOM, timeout, worker restart), the next run restarts from the beginning. For connectors with thousands of sources (large file servers, deep web crawls), this wastes significant time and network I/O.

Current state:

sync starts → process N sources → … crash → next run restarts from last_sync_at (hours ago)

Proposed Solution

Write a checkpoint to Redis after each successfully processed source during sync:

connector:{id}:checkpoint → {
    "started_at": ISO,
    "processed_source_ids": [...],
    "last_checkpointed_at": ISO
}

At sync start, check for an in-progress checkpoint. If found and started within a configurable window (e.g., 2h), resume by skipping already-processed source IDs. Clear the checkpoint on successful completion or explicit full-refresh.

Changes to AbstractConnector:

• _write_checkpoint(source_id) — called after each successful _process_change()
• _read_checkpoint() → set of already-processed source IDs or empty set
• _clear_checkpoint() — called on clean completion
• sync() — skip sources in checkpoint set, write checkpoint after each

Files

• autobot-backend/knowledge/connectors/base.py — add checkpoint methods; update sync() loop
• autobot-backend/knowledge/connectors/models.py — add resumed_from_checkpoint: bool to SyncResult
• autobot-backend/api/knowledge_connectors.py — expose checkpoint status in history entries

Acceptance Criteria

• sync() writes a checkpoint to Redis after each successfully processed source
• On restart, sync reads checkpoint and skips already-processed sources
• SyncResult includes resumed_from_checkpoint: bool
• Checkpoint is cleared on successful full completion
• Full-refresh (incremental=False) always ignores and clears any checkpoint
• Checkpoint TTL set to 24h to prevent stale state accumulation
• Unit tests: normal flow, crash-and-resume, full-refresh override