Add task priority field

Introduce a rank-style priority on tasks, scoped by
(organization_id, state). New tasks auto-assign the next
priority. Reordering uses the same CTE-based algorithm as
trust center references and compliance external URLs.
Exposed through GraphQL, MCP, and the PRIORITY order field.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/coredata/migrations/20260326T160000Z.sql

@@ -0,0 +1,19 @@
+ALTER TABLE tasks ADD COLUMN priority INTEGER;
+
+WITH ranked_tasks AS (
+    SELECT
+        id,
+        ROW_NUMBER() OVER (PARTITION BY organization_id, state ORDER BY created_at DESC, id DESC) AS rn
+    FROM tasks
+)
+UPDATE tasks t
+SET priority = rt.rn
+FROM ranked_tasks rt
+WHERE t.id = rt.id;
+
+ALTER TABLE tasks ALTER COLUMN priority SET NOT NULL;
+
+ALTER TABLE tasks
+ADD CONSTRAINT tasks_organization_id_state_priority_key
+    UNIQUE (organization_id, state, priority)
+    DEFERRABLE INITIALLY DEFERRED;