feat: add 'cancelled' task status; fix 500 on invalid status/priority

TaskStatus enum was missing 'cancelled' — the LLM tried to use it and
hit TaskStatus("cancelled") raising ValueError → 500. Added the value,
a migration to extend the task_status Postgres enum, and proper 400
validation guards on both create and update task routes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

alembic/versions/0031_add_cancelled_task_status.py

@@ -0,0 +1,18 @@
+"""Add 'cancelled' value to task_status enum."""
+
+from alembic import op
+
+revision = "0031"
+down_revision = "0030"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TYPE task_status ADD VALUE IF NOT EXISTS 'cancelled'")
+
+
+def downgrade() -> None:
+    # PostgreSQL does not support removing enum values without a full type rebuild.
+    # Downgrade is a no-op; the value simply becomes unused.
+    pass