Add is_preemptible column to SessionRow

[HyeockJinKim]

Add is_preemptible boolean column to SessionRow with default True. This allows per-session opt-out from preemption independently of priority value.

Preemption condition: is_preemptible == True AND priority <= preemptible_priority (from ScalingGroupOpts).

Scope

• Add is_preemptible: bool column to SessionRow (default True, nullable=False)
• Alembic migration for new column
• Add is_preemptible parameter to session creation flow (registry.py, services/session/)
• Expose in GraphQL session type and session creation input
• Pass through scheduler repository types (PendingSessionData, SessionWorkload)

Key Files

• models/session/row.py — new column
• models/alembic/versions/ — new migration
• registry.py — create_session parameter
• services/session/actions/create_from_params.py
• repositories/scheduler/types/session.py — PendingSessionData
• api/gql/session/types.py — GraphQL field

Success Criteria

• SessionRow.is_preemptible column exists with default True
• Alembic migration applies cleanly (upgrade + downgrade)
• Session creation without is_preemptible parameter defaults to True
• Session creation with is_preemptible=False stores correctly
• GraphQL query returns is_preemptible field for sessions
• Existing sessions (before migration) get default True
• pants lint/check passes for affected packages
• pants test passes for affected packages

JIRA Issue: BA-4912