docs: update database-jobs README for v2 API

[pyramation]

Summary

Updates the database-jobs README to reflect the v2 API changes merged in #61 and #62:

• Removed db_id parameter from all add_job and add_scheduled_job examples (now read from JWT claims internally)
• Added actor_id and is_available to the Jobs Table field list
• Updated queue_name default from "random UUID" to NULL
• Added Performance section documenting partial covering indexes
• Updated add_job and add_scheduled_job parameter lists (no more db_id)
• Added remove_job and force_unlock_workers to function reference
• Updated trigger documentation (statement-level NOTIFY)

Review & Testing Checklist for Human

Risk: green (documentation only)

• Verify the add_job parameter list matches the actual function signature in deploy/schemas/app_jobs/procedures/add_job.sql
• Confirm the index names and definitions in the Performance section match the actual SQL

Notes

• Part of the jobs v2 documentation update across all repos
• See planning issue: https://github.com/constructive-io/constructive-planning/issues/779"

Link to Devin session: https://app.devin.ai/sessions/88b82416d018482791c791cdc91118de
Requested by: @pyramation

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