fix: batch notification queries to eliminate N+1 cascading

[ionfwsrijan]

Description

The notification delivery pipeline performed a cascading N+1 query pattern: for each finding, it fetched all rules, and for each rule it checked delivery history and recorded it — resulting in 1 + N×(1 + 2M) DB queries. With 50 findings and 5 rules, this was 551 DB queries plus 250 outbound HTTP calls in a single synchronous chain.

Changes

1. backend/secuscan/notification_service.py:process_task_notifications() — Rewritten to batch all findings and all active rules upfront (2 queries). Evaluates (finding, rule) pairs in-memory. Dedup check uses a single batched query for all existing delivery history.
2. backend/secuscan/notification_service.py:_deliver_rule_batch() — New helper that sends a single webhook per (rule, task) containing an array of findings instead of one webhook per (finding, rule). Reduces HTTP calls from N×M to M.
3. backend/secuscan/executor.py:execute_task() — Notification dispatch is offloaded to asyncio.create_task() so slow webhook targets don't block task finalization.
4. process_finding_notifications() and deliver_via_rule() are preserved for backward compatibility.

Impact

• DB queries reduced from 1 + N×(1 + 2M) to 3 + M (findings + rules + dedup + M deliveries)
• Webhook HTTP calls reduced from N×M to M (one per rule for all matching findings)
• Task completion is no longer blocked by slow webhook targets
• New webhook payload uses event: finding.alert.batch with a findings array

Files Changed

• backend/secuscan/notification_service.py — Major rewrite of process_task_notifications, new _deliver_rule_batch helper
• backend/secuscan/executor.py — 1 line changed (async dispatch via create_task)

Closes #1625

[ionfwsrijan]

@utksh1 Please review this. The failing check is pre-exisiting

[utksh1]

The N+1 query batching is a solid improvement, but backend-unit CI is currently failing. Please run pytest locally, fix the failing tests, and push again.

[ionfwsrijan]

@utksh1 Please review this now