fix: double booking instructor err

[CK-7vn]

Pre-Submission PR Checklist

• No debug/console/fmt.Println statements
• Unnecessary development comments removed
• All acceptance criteria verified
• Functions according to ticket spexifications
• Tested manually where applicable
• Branch rebased with latest main
• No business logic exists within the database layer

Description of the change

There was an issue with double-booking instructors, and the eligibility of instructors during assignment. This PR implements a double-booking detection system similar to double-booking rooms.

• New database/instructor_conflicts.go: expands an instructor's events/overrides into time slots (getInstructorBookingsInRange) and
  detects overlaps against a candidate schedule, honoring per-occurrence instructor overrides, cancellations, and facility timezone.

  • Refactored rooms.go to share the recurrence-window + overlap + class-name logic (prepareConflictWindow,
    buildConflictsFromBookings) between room and instructor checks (DRY). LockAndCheckConflicts now locks and checks room and/or
    instructor; conflicts are tagged with ConflictType.

  • Wired into handleCreateClass, handleUpdateClass (the ticket's "change a class's instructor" path), and handleCreateEvent; returns
    HTTP 409 with a type-aware message (instructor vs room). Added instructor-belongs-to-facility validation on the event path.

  • Frontend: EditClassModal shows an instructor-aware conflict heading; RoomConflict type gains conflict_type.

  • Tests: new instructor_conflict_test.go covering overlap detection, non-overlap, and the 409-on-instructor-change scenario.

  • Related issues: https://app.asana.com/1/1201607307149189/project/1209460078641109/task/1214430813078163?focus=true

• Related issues: Link to related Asana ticket that this closes.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 76550ac and 81c373e.

📒 Files selected for processing (10)

• backend/src/database/instructor_conflicts.go
• backend/src/database/program_classes.go
• backend/src/database/rooms.go
• backend/src/handlers/class_events.go
• backend/src/handlers/classes_handler.go
• backend/src/handlers/server.go
• backend/src/models/facilities.go
• backend/tests/integration/instructor_conflict_test.go
• frontend/src/pages/class-detail/EditClassModal.tsx
• frontend/src/types/facility.ts

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