feat(constraints): add prefix_tablename option to Unique and Check constraints (#1437)

[shadialhasan]

Description

Fixes #1437

When multiple Table classes inherit a Unique constraint from a common mixin, PostgreSQL requires index names across tables in the same schema to be unique; attempting to apply identical index names results in
elation already exists.

This PR implements the solution suggested in #1437:

1. Adds prefix_tablename: bool = False to Constraint, Unique, and Check constraints.
2. Ensures Constraint instances (and their column references) are safely cloned in TableMetaclass so inheriting tables do not mutate each other's constraint metadata.
3. Automatically prefixes index and constraint names with {tablename}_{base_name} when prefix_tablename=True is enabled.
4. Preserves 100% backward compatibility: prefix_tablename defaults to False, leaving existing constraint naming and migration serialized params byte-identical unless explicitly enabled.
5. Adds comprehensive unit test coverage in ests/table/test_constraints.py and ests/apps/migrations/auto/test_schema_differ.py.

Checks

• Tested with SQLite & schema differ test suites
• Passed lake8 lint checks
• Fully backward-compatible