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

server/migrations/seed/NetcafeDefaults.sql

@@ -0,0 +1,15 @@
+BEGIN;
+
+TRUNCATE public.cafebonus;
+
+INSERT INTO public.cafebonus (time_req, item_type, item_id, quantity)
+VALUES
+    (1800, 17, 0, 50),
+    (3600, 17, 0, 100),
+    (7200, 17, 0, 200),
+    (10800, 17, 0, 300),
+    (18000, 17, 0, 350),
+    (28800, 17, 0, 500),
+    (43200, 17, 0, 500);
+
+END;