rework database to use json string instead of dataclass dump. Prevents failed decodings, makes contents searchable, if needed

src/backend/db.py

@@ -17,7 +17,7 @@ CREATE_TABLE_APPARAT = """CREATE TABLE semesterapparat (
   )"""
 CREATE_TABLE_MEDIA = """CREATE TABLE media (
     id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
-    bookdata BLOB,
+    bookdata TEXT,
     app_id INTEGER,
     prof_id INTEGER,
     deleted INTEGER DEFAULT (0),