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

.github/workflows/go.yml

@@ -81,7 +81,6 @@ jobs:
           ./www/
           ./savedata/
           ./bin/
-          ./bundled-schema/
         retention-days: 7
 
     - name: Build Windows-amd64
@@ -97,7 +96,6 @@ jobs:
           ./www/
           ./savedata/
           ./bin/
-          ./bundled-schema/
         retention-days: 7
 
   lint: