fix(ci): update references from db/schema.sql to sqlx migrations

CI and Docker builds broke after removing db/ in favor of migrations/.
- ci.yml: point psql init at migrations/20260307000000_initial_schema.sql
- Dockerfile: COPY migrations instead of db; remove unused db copy in final stage
- docker-build.yml: remove db/schema.sql presence check

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -56,7 +56,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
 
       - name: Initialize test database
-        run: psql -h localhost -U postgres -d oxicloud_test -f db/schema.sql
+        run: psql -h localhost -U postgres -d oxicloud_test -f migrations/20260307000000_initial_schema.sql
         env:
           PGPASSWORD: postgres