Add forward-planned session scheduling

Summary

DigiScript currently creates a ShowSession the instant "Start" is pressed. This feature adds the ability to schedule sessions ahead of time (with a date, time, and script revision), so that upcoming performances exist as first-class objects before they go live. Sessions can then be prepared (casting assigned, mics reviewed) before show day. The existing "Start Ad-hoc Session" path is preserved.

Part 1 of 3 for issue #792. Must be completed before #1113 (casting matrix) and #1114 (mic re-runs).

Note: Performance categorisation (Preview, Press Night, etc.) is handled via the existing user-defined SessionTag system — no new session_type field is needed. The "Schedule Session" modal includes a tag picker so tags can be assigned at creation time.

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Backend changes

ShowSession model (server/models/session.py):

• Add scheduled_date_time: Mapped[datetime | None] — when set and start_date_time is null, the session is in "scheduled" state

Alembic migration: alembic revision --autogenerate -m "add_scheduled_session_fields" — adds scheduled_date_time column to showsession.

Schema (server/schemas/schemas.py): add scheduled_date_time to ShowSessionSchema.

New/updated controllers (server/controllers/api/show/session/):

Method	Path	Role	Description
POST	/api/v1/show/sessions/schedule	EXECUTE	Create a scheduled session. Body: {scheduled_date_time, tag_ids, script_revision_id}. start_date_time left null. tag_ids assigns existing SessionTags at creation time.
PATCH	/api/v1/show/sessions/{id}	EXECUTE	Update scheduled_date_time, script_revision_id, or tags. Returns 409 if session already started.
DELETE	/api/v1/show/sessions/{id}	EXECUTE	Delete a scheduled session. Returns 409 if already started.

Update SessionStartController (POST /show/sessions/start):

• Accept optional show_session_id in request body
• If present: find that ShowSession, set start_date_time, client_internal_id, user_id (no new row created)
• If absent: existing ad-hoc behaviour unchanged

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Frontend changes (client-v3/)

ConfigSessions.vue — significant update:

• Add "+ Schedule Session" button → ScheduleSessionModal.vue
• Keep "Start Ad-hoc Session" button (existing flow)
• Update table columns: Date · Time | Tags | Revision | Status | Actions
• Status badges: Scheduled (blue) / Next Up (green) / In Progress (green) / Complete (grey)
• Per-row actions: [Edit] [Delete] [Start] for scheduled; [Stop] for live; [View Log] for complete

New ScheduleSessionModal.vue (components/show/config/sessions/):

• Fields: datetime-local picker, tags multi-select (from existing SessionTags), script revision select
• Calls POST /api/v1/show/sessions/schedule

StartSessionConfirmModal.vue — extend to accept show_session_id for starting a pre-scheduled session; show session date/time + tags in header.

stores/show.ts additions:

• New actions: scheduleSession(data), updateScheduledSession(id, data), deleteScheduledSession(id)
• New getter: scheduledSessions — sessions where start_date_time is null, sorted by scheduled_date_time

src/types/api/session.ts: add scheduled_date_time: string | null to ShowSession.

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E2E tests

Update client-v3/e2e/tests/12-show-config-sessions.spec.ts:

• Schedule a session → verify "Scheduled" row appears
• Edit → verify table updates
• Delete → verify row removed
• Start scheduled session → confirm modal → "In Progress" status

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Verification

• pytest — all existing tests pass + new tests for schedule/edit/delete/start-scheduled endpoints
• npm run typecheck + npm run lint in client-v3/
• npm run test:e2e — full suite

Start with server/models/session.py, server/schemas/schemas.py, and the controllers under server/controllers/api/show/session/; inspect the existing SessionStartController and migration conventions. Then trace ConfigSessions.vue, ScheduleSessionModal.vue, stores/show.ts, and the listed E2E spec. Done means scheduled sessions can be created, edited, deleted, prepared, and started without changing ad-hoc behavior, with backend, typecheck, lint, and E2E tests passing.