Add dynamic search and filtering to account activity

[maebeale]

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

• Account activity on the user show page was a static paginated table requiring full page reloads
• Adds real-time search and filtering without leaving the page
• Makes it easy to find specific events in a user's activity history

How did you approach the change?

• Extracted account activity table into a lazy-loading Turbo Frame partial (users/sections/_account_activity)
• Added text search that filters by event name and properties (JSON content)
• Added a "By" dropdown to filter events by the user who performed them (admins + the account owner)
• Reused existing collection Stimulus controller for debounced auto-submit and blur-on-submit loading feedback
• Extracted user_auth_events_base private method to DRY up the Ahoy event query used by both the turbo frame branch and the status/audit queries
• Added "See all user activity" link to the admin activities page filtered by user

Anything else to add?

• No new JavaScript — the existing collection_controller.js handles text input debounce, select change auto-submit, clear-and-submit, and blur-on-submit
• Pagination preserves search and filter params

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