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/models/user.py

@@ -1,35 +0,0 @@
-from dataclasses import asdict, field, dataclass
-import json
-
-
-@dataclass(slots=True)
-class User:
-    id: int
-    image: str
-    password: str
-    username: str
-    roles: list[str]
-    extra: dict[str, str] = field(default_factory=dict)
-
-    # NOTE: roles: ['admin', 'user', 'curator']
-    roles: list[str] = field(default_factory=lambda: ["user"])
-
-    def todict(self):
-        this_dict = asdict(self)
-        del this_dict["password"]
-
-        if type(this_dict["roles"]) is str:
-            # INFO: this is an attempt to fix string roles!
-            try:
-                this_dict["roles"] = json.loads(this_dict["roles"])
-            except json.JSONDecodeError:
-                this_dict["roles"] = []
-
-        return this_dict
-
-    def todict_simplified(self):
-        return {
-            "id": self.id,
-            "username": self.username,
-            "firstname": self.extra["firstname"] if self.extra else "",
-        }