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.
Replace 4 independent schema management code paths (Docker shell
script, setup wizard pg_restore, test helpers, manual psql) with a
single migration runner embedded in the server binary.
The new server/migrations/ package uses Go embed to bundle all SQL
schemas. On startup, Migrate() creates a schema_version tracking
table, detects existing databases (auto-marks baseline as applied),
and runs pending migrations in transactions.
Key changes:
- Consolidated init.sql + 9.2-update + 33 patches into 0001_init.sql
- Setup wizard simplified to single "Apply schema" checkbox
- Test helpers use migrations.Migrate() instead of pg_restore
- Docker no longer needs schema volume mounts or init script
- Seed data (shops, events, gacha) embedded and applied via API
- Future migrations just add 0002_*.sql files — no manual steps
