Add task priority enum and rename priority to rank

The existing integer priority field represents positional ordering
within a state, not semantic importance. Rename it to rank and
introduce a new priority field with enum values URGENT, HIGH,
MEDIUM and LOW across the entire stack.

Rank is now scoped to (state, priority) so tasks are ordered
within each priority group. A generated priority_rank column
combines both fields into a single sortable integer for cursor
pagination.

Dragging a task across priority groups updates its priority
automatically based on the drop position neighbors. The backend
first moves the task to the new group then repositions it at the
target rank.

The migration defaults existing rows to MEDIUM priority and
backfills ranks per (state, priority) group.

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

pkg/coredata/migrations/20260330T120000Z.sql

@@ -0,0 +1,36 @@
+-- Rename priority to rank
+ALTER TABLE tasks RENAME COLUMN priority TO rank;
+
+ALTER TABLE tasks DROP CONSTRAINT tasks_organization_id_state_priority_key;
+
+-- Add task priority enum
+CREATE TYPE task_priority AS ENUM ('URGENT', 'HIGH', 'MEDIUM', 'LOW');
+
+ALTER TABLE tasks ADD COLUMN priority task_priority NOT NULL DEFAULT 'MEDIUM'::task_priority;
+
+ALTER TABLE tasks ALTER COLUMN priority DROP DEFAULT;
+
+-- Rank is now scoped to (state, priority) — backfill ranks per group
+WITH ranked AS (
+    SELECT id, ROW_NUMBER() OVER (
+        PARTITION BY organization_id, state, priority
+        ORDER BY rank
+    ) AS new_rank
+    FROM tasks
+)
+UPDATE tasks SET rank = ranked.new_rank FROM ranked WHERE tasks.id = ranked.id;
+
+ALTER TABLE tasks
+ADD CONSTRAINT tasks_organization_id_state_priority_rank_key
+    UNIQUE (organization_id, state, priority, rank)
+    DEFERRABLE INITIALLY DEFERRED;
+
+-- Computed column for composite ordering (priority level then rank)
+ALTER TABLE tasks ADD COLUMN priority_rank int GENERATED ALWAYS AS (
+    (CASE priority
+        WHEN 'URGENT' THEN 1
+        WHEN 'HIGH' THEN 2
+        WHEN 'MEDIUM' THEN 3
+        WHEN 'LOW' THEN 4
+    END) * 1000000 + rank
+) STORED;