Fix capacity handling for duplicate External slots

[eduardpiazza]

Summary

• treat each External source event occurrence as an independent capacity unit, identified by source object URI plus occurrence start/end
• persist that identity on destination bookings as X-APPT-SOURCE
• consume only the matching source occurrence when a booking exists, so booking one of 100 duplicate slots leaves 99 available
• keep legacy, manual, moved, or source-missing destination events as normal busy-time blockers
• apply the same source-aware logic during the booking re-check to retain race-condition protection

This intentionally scopes the change to External mode. Weekly Template capacity is not changed here.

Addresses the External-mode capacity case discussed in #668. Related to #54.

Tests

• ExternalSlotCapacityTest: 6 tests, 18 assertions
• PHP syntax checks for all changed PHP files
• composer validate --no-check-publish
• git diff --check

The complete repository unit bootstrap requires a full Nextcloud server checkout because tests/bootstrap.php loads ../../../lib/base.php; that server checkout is not present in this standalone app checkout.