feat(db): add embedded auto-migrating schema system

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

Dockerfile

@@ -16,7 +16,6 @@ RUN adduser -D -h /app erupe
 WORKDIR /app
 
 COPY --from=builder /build/erupe-ce .
-COPY --from=builder /build/schemas/ ./schemas/
 
 # www/ and bin/ are mounted at runtime if needed