refactor: share instance ids, booking slot helpers, and guest urls

[cursor]

What and why

Behavior-preserving cleanup of recent provider and booking-reschedule merges after #3125:

• Share recurring-instance id mint and parse between Google and Apple so sparse cancellations stay byte-aligned
• Share the access-role capability table used by Google and Apple calendar discovery
• Clamp connection event and busy queries with syncHorizon instead of recomputing the window
• Share public booking month-slot query, prefetch, and next-available walks between book and reschedule
• Extract the guest alert and sticky-step class; inline two thin copy-link wrappers
• Pair cancel and reschedule guest-action URLs once in the public booking service

Left alone: WelcomeModal step machine, Block Party HUD, Google toast defer scaffolding, OAuth token helpers (transport still differs), slot-picker flow hooks (confirm vs create still diverge), and CalDAV discovery fixtures.

Verify

VERDICT: PASS

Checks run: test:sync:fast, test:web, test:backend:fast, type-check, lint, knip, test:a11y, test:e2e

  

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Note

Low Risk
Refactor consolidates duplicate logic without changing contracts; the recurring-instance-id and horizon clamp paths are sensitive but intended to be byte-identical to prior behavior.

Overview
Behavior-preserving cleanup that deduplicates cross-cutting logic across sync providers, connection APIs, public booking backend, and guest UI.

Sync: New shared recurring-instance-id mint/parse powers Google and Apple instance IDs so sparse cancellations stay aligned; Apple’s writer now imports parseAppleInstanceId from apple-instance-id instead of inlining it. Calendar discovery for Google and Apple both use capabilitiesForAccessRole. Event and busy queries clamp ranges via syncHorizon in a single clampQueryToHorizon helper. Apple discovery errors pass CalDAV reasons through without an extra mapper.

Backend booking: guestActionUrls builds cancel and reschedule links once for create, patch, and reschedule.

Web booking: Month-slot queries, prefetch, and “jump to next available” share one code path for book vs reschedule flows. UI extracts PublicBookingAlert and exports sticky-step styling from the layout; thin cancel/reschedule copy-link wrappers are removed in favor of PublicBookingCopyGuestAction.

^{Reviewed by Cursor Bugbot for commit d7b7b20. Bugbot is set up for automated code reviews on this repo. Configure here.}

[tyler-dane]

Superseded by #3446, which rebases these helpers onto current main and adds the Microsoft Graph fetch share. Closing this conflicting PR so merge-guard does not try both.