feat: add SQL migration system using sqlx::migrate!()

Replace manual schema.sql application with sqlx's built-in migration
system. Migrations are embedded at compile time and tracked in the
_sqlx_migrations table. Pending migrations run automatically on startup.

- Move db/schema.sql → migrations/20260307000000_initial_schema.sql
- Remove apply_schema() and split_sql_statements() from db.rs
- Add run_migrations() using sqlx::migrate!() macro
- Remove docker-compose schema.sql mount (app handles it now)
- Enable sqlx "migrate" feature in Cargo.toml

Future schema changes: add a new timestamped .sql in migrations/.

Closes #190

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

docker-compose.yml

@@ -12,7 +12,6 @@ services:
       - oxicloud
     volumes:
       - pg_data:/var/lib/postgresql/data
-      - ./db/schema.sql:/docker-entrypoint-initdb.d/10-schema.sql
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U postgres"]
       interval: 5s