feat(scheduler): admin-editable job intervals (DB-backed, live reschedule)

Add scheduler_settings (per-job interval override, migration 0015) and a
PATCH /api/admin/scheduler/jobs/{id} that persists the override and live-
reschedules the running APScheduler job. Intervals load from the DB (env
defaults as fallback); the snapshot reports the live trigger interval.

backend/alembic/versions/0015_scheduler_settings.py

@@ -0,0 +1,30 @@
+"""scheduler job interval overrides
+
+Revision ID: 0015_scheduler_settings
+Revises: 0014_demo_account
+Create Date: 2026-06-16
+
+Adds scheduler_settings: a per-job run-interval override (minutes), editable from the admin
+Scheduler dashboard. Absent row = use the env/config default, so this is purely additive.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0015_scheduler_settings"
+down_revision: Union[str, None] = "0014_demo_account"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "scheduler_settings",
+        sa.Column("job_id", sa.String(length=40), primary_key=True),
+        sa.Column("interval_minutes", sa.Integer(), nullable=False),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("scheduler_settings")