Add scope column for categorising feature flags

[stevethomas]

Summary

Re-opened from #6. Originally merged but reverted from main to keep v1.x clean — this is targeted at v2.

• Adds scope and description columns to the features table
• Supports rich config format alongside simple FeatureState values
• Backed enum support for scope values
• Sync action updates scope/description on re-sync without touching state

Test plan

• Existing tests pass
• New scope/description tests pass
• Migration runs cleanly on fresh database

[sentry]

Bug: The normalise method accesses keys on the $config variable without first validating that it is an array and contains required keys, risking a crash during synchronization.
_{Severity: HIGH}

Suggested Fix

Before accessing array keys on the $config variable within the normalise method, add a validation step. Check that $config is an array and that it contains the required 'state' key. If the validation fails, throw a descriptive InvalidArgumentException to inform the user of the specific configuration error.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/Actions/SyncFeaturesAction.php#L66

Potential issue: In the `SyncFeaturesAction::normalise` method, the `$config` variable,
which comes directly from user configuration, is accessed as an array without prior
validation. If a user provides a rich-format configuration as an array but omits the
required `'state'` key, the access on line 66 (`$config['state']`) evaluates to `null`.
This `null` value is then passed downstream, causing an `InvalidArgumentException` in
the `FeatureStateCast::set` method, which crashes the entire feature flag
synchronization action. A similar fatal error occurs if the config value is not an array
at all (e.g., a string or `null`), as the code will attempt an illegal offset access.

_{Did we get this right? 👍 / 👎 to inform future reviews.}