Reorganize repository structure for better organization

- Move backend code to swingmusic/ folder
- Move client applications to root level (swingmusic-android, swingmusic-desktop, swingmusic-webclient)
- Remove intermediate backend/ and clients/ folders
- Update README with new folder structure and setup instructions
- Clean and organized repository layout

src/swingmusic/db/metadata.py

@@ -1,35 +0,0 @@
-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