Add responded checkbox to system notifications index

[maebeale]

What is the goal of this PR and why is this important?

• Staff need a way to track which contact-us submissions have been replied to.
• Currently there's no signal in the notifications index — admins have to remember (or open each notification) to know what's outstanding.

How did you approach the change?

• Added a responded:boolean column to notifications (default false).
• New "Responded" column on the system notifications index renders an inline-editable checkbox only for contact_us / contact_us_fyi kinds; other rows show an em-dash so staff aren't tempted to mark unrelated notifications.
• The checkbox uses the existing autosave Stimulus controller, so toggling fires a PATCH to notifications#update — no submit button, no page reload.
• update? policy is admin-only (matches the index permission).
• Checkbox uses accent-green-600 so the native check renders green.

UI Testing Checklist

• Sign in as admin → visit /notifications
• Filter to a contact_us_fyi row → checkbox renders unchecked
• Tick the checkbox → green ring flashes (autosave success), refresh → still checked
• Untick → state persists after refresh
• Non-contact-us rows show "—" in the Responded column
• Sign in as a non-admin → PATCH /notifications/:id is denied

Anything else to add?

• Migration is reversible (explicit up/down with if_exists: true).
• Started with a boolean rather than responded_at datetime — YAGNI; we can layer a timestamp later if audit needs it.

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