Move backend files to root level for cleaner GitHub display

- Move all backend files from swingmusic/ to root level
- Backend files now display directly on GitHub repository page
- Keep client applications as submodules (swingmusic-android, swingmusic-desktop, swingmusic-webclient)
- Update README to reflect new structure (no cd swingmusic needed)
- Cleaner, more professional GitHub repository layout

Files moved to root:
- src/ (main source code)
- pyproject.toml, requirements.txt, run.py
- swingmusic.spec, uv.lock, version.txt
- services/

Result: GitHub shows backend files directly while maintaining organized structure

src/swingmusic/db/metadata.py

@@ -0,0 +1,35 @@
+from swingmusic.db import Base
+
+
+from sqlalchemy import Integer, insert, select, update
+from sqlalchemy.orm import Mapped, mapped_column
+
+from swingmusic.db.engine import DbEngine
+
+
+class MigrationTable(Base):
+    __tablename__ = "dbmigration"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    version: Mapped[int] = mapped_column(Integer())
+
+    @classmethod
+    def set_version(cls, version: int):
+        with DbEngine.manager(commit=True) as conn:
+            result = conn.execute(
+                update(cls).where(cls.id == 1).values(version=version)
+            )
+
+            if result.rowcount == 0:
+                conn.execute(insert(cls).values(id=1, version=version))
+
+    @classmethod
+    def get_version(cls):
+        with DbEngine.manager() as conn:
+            result = conn.execute(select(cls.version).where(cls.id == 1))
+            result = result.fetchone()
+
+            if result:
+                return result[0]
+
+            return -1