Add position field to multi_panel_targets table

[biast12]

Introduces a 'position' column to the multi_panel_targets table with a unique constraint per multi_panel_id. Updates queries and the Insert method to handle position, and ensures results are ordered by position.

For PR: TicketsBot-cloud/dashboard#63

Migration query:

-- Add position column if it doesn't exist
ALTER TABLE multi_panel_targets
ADD COLUMN IF NOT EXISTS position int NOT NULL DEFAULT 0;

-- Update existing records to set position based on panel_id order
-- This is a best-effort ordering for existing data
WITH ranked_panels AS (
    SELECT
        multi_panel_id,
        panel_id,
        ROW_NUMBER() OVER (PARTITION BY multi_panel_id ORDER BY panel_id) - 1 AS new_position
    FROM multi_panel_targets
)
UPDATE multi_panel_targets
SET position = ranked_panels.new_position
FROM ranked_panels
WHERE multi_panel_targets.multi_panel_id = ranked_panels.multi_panel_id
  AND multi_panel_targets.panel_id = ranked_panels.panel_id
  AND multi_panel_targets.position <> ranked_panels.new_position;

-- Add unique constraint to ensure each position is unique within a multi-panel
DO $$
BEGIN
    IF NOT EXISTS (
        SELECT 1 FROM pg_constraint
        WHERE conname = 'multi_panel_targets_multi_panel_id_position_key'
    ) THEN
        ALTER TABLE multi_panel_targets
        ADD CONSTRAINT multi_panel_targets_multi_panel_id_position_key
        UNIQUE (multi_panel_id, position);
    END IF;
END $$;