Persist per-project schedules for scan execution policies

What does this MR do and why?

Persist per-project schedules for scan execution policies

Add the security_scan_execution_project_schedules table and ScanExecutionProjectSchedule model to persist per-project random offsets for scheduled scan execution policies.

The RuleScheduleService now creates a project schedule row on every execution (behind the scan_execution_policy_project_schedule_creation feature flag).

References

1. 👉 Persist per-project schedules for scan executio... (!227096 - merged)
2. Add per-project scheduling for scan execution p... (!227101)
3. Draft: Sync schedules when policy changes (!227102)

Database

.for_rule_schedule

SELECT "security_policies".* FROM "security_policies" WHERE "security_policies"."type" = 1 AND "security_policies"."security_orchestration_policy_configuration_id" = 309 AND "security_policies"."policy_index" = 0

Plan: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/49791/commands/148219

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:scan_execution_policy_project_schedule_creation)

2. Create a project with a scan execution policy that runs on schedule

   scan_execution_policy:
     - name: Secrets on schedule
       description: ''
       enabled: true
       actions:
         - scan: secret_detection
           template: latest
           variables:
             SECURE_ENABLE_LOCAL_CONFIGURATION: 'false'
       rules:
         - type: schedule
           cadence: 0 0 1 * *
           branch_type: default
           timezone: Europe/Zurich
       skip_ci:
         allowed: true
   

3. Trigger the schedule pipeline manually via rails console

   schedule = Security::OrchestrationPolicyRuleSchedule.last
   schedule.update_columns(next_run_at: 1.day.ago) && Security::OrchestrationPolicyRuleScheduleWorker.new.perform    

4. Verify that project schedule record was created with schedule.project_schedules
5. Run the schedule again and verify that it doesn't get re-created again

MR acceptance checklist

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Related to #592731