refactor: don't blindly create all tables in the beginning

The only table that has been added since DB version 6 (the earliest supported version), is the versions table in DB version 101.
This commit removes the "create all tables" statement, and instead creates the versions table in the 101 migration.
See 12e074b71d.

src/tagstudio/core/library/alchemy/library.py

@@ -444,6 +444,7 @@ class Library:
 
             # TODO - find a better way
             # is this the better way?
+            # Could we perhaps update the row we are reading from here?
             result = conn.execute(text("SELECT SEQ FROM sqlite_sequence WHERE name='tags'"))
             autoincrement_val = result.scalar()
             if not autoincrement_val or autoincrement_val <= RESERVED_TAG_END:
@@ -565,19 +566,6 @@ class Library:
             self.save_library_backup_to_disk()
             self.library_dir = None
 
-        # TODO: this is very sketchy; blindly creating all tables the newest DB version should have
-        # without considering what version the DB is currently on and then doing all of the
-        # migrations after that seems like it could cause problems in some scenarios.
-        # instead only have this on creation and create new tables as part of migrations
-        # Note: this actually produces an error and fails to initialise built-in tags when opening
-        # a library that doesn't yet have the is_hidden property on the tags table
-        logger.info("[Library] Creating DB tables...")
-        with self.engine.connect() as conn:
-            # TODO: this should instead be migrations that create the exact tables
-            # that were added in the respective DB versions
-            ModelBase.metadata.create_all(conn)
-            conn.commit()
-
         # migrate DB step by step from one version to the next
         # (migration_method, db_version, initial_db_version)
         migrations = [
@@ -705,6 +693,16 @@ class Library:
 
     def __apply_db101_migration(self, session: Session, library_dir: Path):
         """Migrate DB to DB_VERSION 101."""
+        # Create versions table
+        session.execute(
+            text("""
+        CREATE TABLE versions (
+            "key" VARCHAR NOT NULL PRIMARY KEY,
+            value INTEGER NOT NULL,
+        );
+        """)
+        )
+        session.flush()
         # Ensure version rows are present
         session.add(Version(key=DB_VERSION_INITIAL_KEY, value=100))
         session.flush()