OnSched 3.7.5 - dashboard sign-in and availability fixes

Dashboard

Email sign-in: Dashboard users now receive a one-time sign-in code by email. Enter the code in the Dashboard sign-in page; no URL editing is required. This avoids magic link failures caused by enterprise email security scanners opening links before the user.

Out-of-office blocks: Replacing OOF blocks via POST /v3/unavailability/setUnavailabilityBlocks now clears availability cache only after the database transaction commits, preventing concurrent availability requests from serving stale slots for up to the cache TTL.
GET /v3/availability?syncExternal=true: External calendar sync now runs for all resources linked to the requested service and location when ResourceIds is omitted, matching availability calculation. Updated implicit-resource results also invalidate related service/location cache variants, and requests skip cached availability when any connected calendar cannot be fetched during sync.
External calendar sync: Google and Outlook sync now load existing external unavailability rows that overlap the requested window or match an incoming provider event ID. This fixes stale over-blocking when a multi-day event is shortened and correctly updates events moved from outside into a narrower availability or bulk sync window.
Provider failure safety: Transient Google or Microsoft Graph failures no longer look like an empty calendar, preventing stored busy blocks from being deleted during a failed sync.
POST /v3/resource/{id}/externalCalendar/createUnavailabilityBulk: Bulk sync now ignores calendars with read: false, consistent with immediate availability sync.