T11.1: UsageEvent model + migration (CR-019)

New usage_events table, separate from audit_log - see the model docstring
for why. Verified: applies and downgrades cleanly on SQLite, and the
postgresql-dialect --sql render has no risky defaults (the BL-027 class
of defect). No app wiring yet - that's T11.2.

server/alembic/versions/8e2cb3003f8a_usage_events.py

@@ -0,0 +1,46 @@
+"""usage events (CR-019, wave 11)
+
+Append-only navigation/session activity, separate from audit_log on purpose —
+see the UsageEvent docstring in server/models.py. Retention is indefinite by
+decision (docs/waves/decisions-2026-09-17.md); nothing here schedules a purge.
+
+Revision ID: 8e2cb3003f8a
+Revises: 1d60a608bb51
+Create Date: 2026-09-23 00:00:00.000000
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = '8e2cb3003f8a'
+down_revision = '1d60a608bb51'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table('usage_events',
+    sa.Column('id', sa.String(length=40), nullable=False),
+    sa.Column('at', sa.DateTime(timezone=True), nullable=False),
+    sa.Column('username', sa.String(length=200), nullable=False),
+    sa.Column('project_id', sa.String(length=40), nullable=True),
+    sa.Column('tool', sa.String(length=40), nullable=False),
+    sa.Column('event', sa.String(length=40), nullable=False),
+    sa.Column('detail', sa.JSON(), nullable=False),
+    sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_usage_events_at'), 'usage_events', ['at'], unique=False)
+    op.create_index(op.f('ix_usage_events_username'), 'usage_events', ['username'], unique=False)
+    op.create_index(op.f('ix_usage_events_project_id'), 'usage_events', ['project_id'], unique=False)
+    op.create_index(op.f('ix_usage_events_tool'), 'usage_events', ['tool'], unique=False)
+    op.create_index(op.f('ix_usage_events_event'), 'usage_events', ['event'], unique=False)
+
+
+def downgrade() -> None:
+    op.drop_index(op.f('ix_usage_events_event'), table_name='usage_events')
+    op.drop_index(op.f('ix_usage_events_tool'), table_name='usage_events')
+    op.drop_index(op.f('ix_usage_events_project_id'), table_name='usage_events')
+    op.drop_index(op.f('ix_usage_events_username'), table_name='usage_events')
+    op.drop_index(op.f('ix_usage_events_at'), table_name='usage_events')
+    op.drop_table('usage_events')