Refactor database initialization and remove Alembic migrations

- Removed legacy Alembic migration files and consolidated schema management into a new Pydantic-backed initializer (`scripts/init_db.py`).
- Updated `main.py` to ensure the new DB initializer runs on startup, maintaining idempotency.
- Adjusted session management in `config/database.py` to prevent DetachedInstanceError.
- Introduced new enums in `models/enums.py` for better organization and clarity.
- Refactored various models to utilize the new enums, improving code maintainability.
- Enhanced middleware to handle JSON validation more robustly, ensuring non-JSON requests do not trigger JSON errors.
- Added tests for middleware and enums to ensure expected behavior and consistency.
- Updated changelog to reflect significant changes and improvements.

tests/test_login_form_middleware.py

@@ -0,0 +1,30 @@
+from fastapi.testclient import TestClient
+
+from main import app
+
+
+def test_login_form_post_does_not_trigger_json_error():
+    """POST form-encoded data to /login and assert middleware doesn't return
+    the JSON "Invalid JSON payload" error which indicates the middleware
+    attempted to parse non-JSON bodies.
+    """
+    client = TestClient(app)
+
+    resp = client.post(
+        "/login",
+        data={"username": "no-such-user", "password": "x"},
+        headers={"Accept": "text/html"},
+    )
+
+    content_type = resp.headers.get("content-type", "")
+
+    # If middleware raised the JSON error we'd get an application/json body
+    # with detail == "Invalid JSON payload". Ensure that's not the case.
+    if content_type.startswith("application/json"):
+        body = resp.json()
+        assert body.get("detail") != "Invalid JSON payload", (
+            "Middleware attempted to parse non-JSON body as JSON and failed"
+        )
+
+    # At minimum the endpoint should not error with the JSON payload message.
+    assert True