Create SupportSessionMessage model and migration

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Description

## Task
Create the `SupportSessionMessage` model and database migration for storing chat messages in remote support sessions.

## Schema Design
```ruby
create_table :support_session_messages do |t|
  t.references :support_session, null: false, foreign_key: true, index: true
  t.references :user, null: true, foreign_key: true  # null for guest messages
  t.string :participant_type, null: false  # 'operator' or 'guest'
  t.string :participant_name, null: false  # display name
  t.text :content, null: false
  t.jsonb :metadata, default: {}
  t.timestamps
end

add_index :support_session_messages, [:support_session_id, :created_at]
```

## Model Implementation
- Belongs to `support_session`
- Belongs to `user` (optional - null for guests)
- Enum for `participant_type`: `[:operator, :guest]`
- Validates content presence and length (max 5000 chars)
- Method `to_broadcast_payload` returning hash for frontend consumption

## Files to Create/Modify
- `db/migrate/xxx_create_support_session_messages.rb`
- `app/models/support_session_message.rb`
- `app/models/support_session.rb` (add association)
- `spec/models/support_session_message_spec.rb`
- `spec/factories/support_session_messages.rb`