Add ability to create and view blocking states (#612)

[tungleduyxyz]

Summary

Implements issue #612 — read and write paths for Kill Bill blocking states.

Read path:

• Account Timeline now surfaces blocking state events (previously silently filtered out via next if event.event_type == 'SERVICE_STATE_CHANGE')
• Dedicated "Blocking States" event filter on the timeline alongside existing Subscription/Payment/Billing filters
• CSV export includes blocking state rows
• New "Blocking States" index page per account (accessible via the left sidebar nav)

Write path:

• "Create Blocking State" form for Account, Bundle, and Subscription levels
• Entry points: Account → Billing Information section button; Subscriptions table → row dropdown per bundle/subscription; Blocking States index page → "Add Blocking State" button
• Form fields: Applies To (read-only type + object ID), State (required), Service (required), Block flags — Change / Entitlement / Billing, Effective Date, Comment
• "About Blocking States" info panel beside the form explaining blocking state semantics

Test plan

• Create an account-level blocking state via the Billing Information section button; confirm it appears on the Account Timeline and in the Blocking States index
• Create a bundle-level blocking state via the Subscriptions table row dropdown; confirm it appears correctly typed (BUNDLE) in the Blocking States index
• Create a subscription-level blocking state via the Subscriptions table row dropdown; confirm it appears correctly typed (SUBSCRIPTION) in the Blocking States index
• Verify the Account Timeline "Blocking States" filter shows/hides blocking state events independently of other filters
• Verify the CSV export includes blocking state rows
• Verify "Add Blocking State" button is hidden for users without account:update permission

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