Polish: clearer errors, corrected docstrings, setuptools pin

- insert/upsert into a name that is actually a view now shows a
  clean error instead of a traceback
- db.view() on a name that is a table now says so in its error
- Database.__getitem__ docstring documents that views are returned
  for view names; hash_id docstring corrected (it is a column name,
  not a boolean)
- Registering two migrations with the same name in one set now
  raises ValueError at registration time instead of executing both
  and failing with IntegrityError at apply time
- build-system requires setuptools>=77, needed for the PEP 639
  license expression

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UnLnhsH25Nnv7LHhekUfPd

sqlite_utils/migrations.py

@@ -44,8 +44,15 @@ class Migrations:
         """
 
         def inner(func: Callable) -> Callable:
+            migration_name = name or getattr(func, "__name__")
+            if any(m.name == migration_name for m in self._migrations):
+                raise ValueError(
+                    "Migration '{}' is already registered in set '{}'".format(
+                        migration_name, self.name
+                    )
+                )
             self._migrations.append(
-                self._Migration(name or getattr(func, "__name__"), func, transactional)
+                self._Migration(migration_name, func, transactional)
             )
             return func