⚡ Bolt: Add indexes to processing_jobs to optimize /jobs endpoint

[kourdroid]

💡 What: Added a SQL migration (004_processing_jobs_indexes.sql) to create database indexes on the processing_jobs table. The indexes are (status, created_at DESC) and (created_at DESC).
🎯 Why: The /jobs API endpoint queries the processing_jobs table and orders the results by created_at DESC, optionally filtering by status. Without indexes, these queries result in sequential scans and expensive O(N log N) sorts.
📊 Impact: This optimization turns the O(N log N) sorts into O(LIMIT) B-Tree backward walks. It will significantly reduce the latency of the /jobs endpoint and lower CPU/IO load on the database, particularly as the table grows large.
🔬 Measurement: Execute EXPLAIN ANALYZE on the following query before and after applying the migration: SELECT * FROM processing_jobs WHERE status = 'PENDING' ORDER BY created_at DESC LIMIT 50;. The query plan should shift from Seq Scan + Sort to an Index Scan (or Index Only Scan) on the newly created index.

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PR created automatically by Jules for task 3438726045328796059 started by @kourdroid

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