Add native description column to features with UI support

[markaschneider]

This is a follow-up to the descriptions_source callback added in #461.

Problem

Today, features can be created dynamically through the Flipper UI — no code change needed. But descriptions can only be provided through a descriptions_source lambda configured in code. This creates a mismatch: the person creating a feature
in the UI can't describe what it does in the same workflow. They have to go update a config file, YAML, or database table separately.

As @jnunemaker noted in #461:

I'd rather have native storage of other attributes like description

Proposal

Add a description text column to flipper_features and expose it as an editable field in the UI on both the feature creation form and the feature detail page.

For backward compatibility with descriptions_source, the resolution would be:

1. Load native descriptions from the description column
2. Merge descriptions_source results on top — callback values take precedence when present

This means:

• New users get descriptions out of the box via the UI with zero configuration
• Existing descriptions_source users see no behavior change — their callback still wins
• Both can be used together — native descriptions as defaults, callback overriding specific ones

Scope

• Migration adding description (text, nullable) to flipper_features
• Update the Feature create/edit UI actions to accept and persist description
• Update the feature detail and features list views to display the native description
• Merge logic: native descriptions as the base layer, descriptions_source on top
• show_feature_description_in_list continues to control list page visibility

Happy to submit a PR if this direction sounds right.