fix(migrations): drop unused data column from distribution table (#169)

The distribution table had a `data bytea NOT NULL` column that was never
read by the Go code — item data is stored in distribution_items instead.
The NOT NULL constraint forced dummy values in seed data and test inserts.

Remove the column from the baseline schema, seed data, and tests, and
add migration 0005 to drop it from existing databases.

server/migrations/sql/0001_init.sql

@@ -264,7 +264,6 @@ CREATE TABLE public.distribution (
     max_sr integer,
     min_gr integer,
     max_gr integer,
-    data bytea NOT NULL,
     rights integer,
     selection boolean
 );

server/migrations/sql/0005_distribution_drop_data.sql

@@ -0,0 +1 @@
+ALTER TABLE public.distribution DROP COLUMN IF EXISTS data;