Update channel column to support all communication channels
| Field | Type | Null | Key |
|---|---|---|---|
| id | int(11) | NO | PRI |
| from_type | enum('user','ai') | NO | MUL |
| from_id | varchar(100) | NO | |
| to_type | enum('user','ai') | NO | MUL |
| to_id | varchar(100) | NO | |
| channel | varchar(50) | NO | MUL |
| subject | varchar(255) | YES | |
| message | text | NO | |
| priority | enum('low','normal','high','urgent') | YES | |
| status | enum('sent','delivered','read','responded') | YES | MUL |
| metadata | json | YES | |
| created_at | timestamp | NO | MUL |