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.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Distribution table: remove unused `data bytea NOT NULL` column that prevented seed data from matching Go code expectations (#169)
+
 ### Added
 
 - API: Standardized JSON error responses (`{"error":"...","message":"..."}`) across all endpoints via `writeError` helper