Backend - `EPApprovalRequest` request type

[zzacharo]

Implementation details

• EPApprovalRequest request type (submit/accept/decline actions) with full payload schema
• Accept: auto-generates CERN-EP-{year}-{seq} report number, mints apprn PID, propagates to submitted version and all newer published versions via DB query
• EPReportNumberComponent: guards apprn against removal/change using the PID table as source of truth (catches race condition where draft predates approval); soft field error on update_draft, hard raise on publish; superusers exempt
• Email notifications on submit/accept/decline
• Tests covering submit→accept/decline, report number sequencing, propagation, immutability, race condition