Look into a way to allow for multiple teachers

[bcordis]

A way to allow sermon to show up on multiple teachers streams. Giving the flexibility to add more then one teacher to each sermon.

We may need to us a Join table to allow for SQL optimization.

[bcordis]

Implemented in PR #1069 (merged to development).

What was added:

• New #__bsms_study_teachers junction table linking messages to multiple teachers with ordering
• Admin message edit: repeatable subform replaces single teacher dropdown — add, remove, and reorder teachers
• All 20+ query files (models, helpers, finder plugin) updated to JOIN through junction table with COALESCE fallback
• Teacher filter uses EXISTS subquery to match any teacher on a message
• Layout Editor: new teachers-list element displays all teachers comma-separated
• CwmstudyteacherHelper batch-loads teachers with static cache (same pattern as scriptures)
• Legacy teacher_id column kept and synced from primary teacher (ordering=0) for backward compatibility
• Migration auto-populates junction table from existing data; also handles DB restore path
• Duplicate teacher cleanup: merges same-name teachers, deduplicates aliases, adds UNIQUE KEY