database: Add index to optimize background job queue processing

This commit creates a composite index on `(priority DESC, id ASC)` to improve performance of job selection queries that order by priority and id for queue processing.

This improved the job selection query performance from roughly 800ms to 0.07ms after enqueuing all of the `analyze-crate-file` backfill background jobs.

Author: Turbo87

migrations/2025-09-22-090805_add_background_jobs_priority_id_index/down.sql

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+DROP INDEX CONCURRENTLY background_jobs_priority_id_index;

migrations/2025-09-22-090805_add_background_jobs_priority_id_index/metadata.toml

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+run_in_transaction = false

migrations/2025-09-22-090805_add_background_jobs_priority_id_index/up.sql

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+CREATE INDEX CONCURRENTLY background_jobs_priority_id_index
+    ON background_jobs (priority DESC, id ASC);