Exploratory Google Calendar API integration

[Copilot]

Adds foundational plumbing for syncing training plan workouts to Google Calendar. Follows the existing Strava integration patterns: service object → background jobs → model callbacks.

Data model

• users.google_calendar_id — persistent calendar ID per user
• plans.calendar_sync_enabled — per-plan sync toggle
• activities.google_calendar_event_id — tracks synced events

GoogleCalendarService

Handles all Calendar API interactions via google-apis-calendar_v3: find/create a dedicated "Lace Training" calendar, CRUD events as all-day entries, and refresh Google OAuth tokens via Signet.

service = GoogleCalendarService.new(user)
service.sync_plan(plan)         # bulk sync all activities
service.update_event(activity)  # single event upsert
service.delete_event(cal_id, event_id)

Background jobs

• SyncPlanToCalendarJob — bulk sync on enable
• RemovePlanCalendarEventsJob — bulk remove on disable
• UpdateCalendarEventJob — triggered by activity after_update
• DeleteCalendarEventJob — triggered by activity before_destroy

Model callbacks

• Plan: after_update enqueues sync/remove when calendar_sync_enabled changes
• Activity: after_update enqueues event update on distance/description/date change; before_destroy enqueues event deletion

OAuth scope

Google OAuth now requests https://www.googleapis.com/auth/calendar with access_type: "offline".

UI

Toggle button on plan show page, visible only when user has a connected Google account.

Notes

• User#has_google_access_token? uses parameterized raw SQL to check token presence without triggering ActiveRecord encryption decryption — works around a pre-existing issue where test env lacks encryption credentials.
• 24 new tests covering service, jobs, and model callbacks.

Original prompt

Make an exploratory PR for Google Calendar API integration

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