feat(message): add message center module (Phase 5)

Implement the complete message center with:
- Database migrations for message_templates, messages, message_subscriptions tables
- erp-message crate with entities, DTOs, services, handlers
- Message CRUD, send, read/unread tracking, soft delete
- Template management with variable interpolation
- Subscription preferences with DND support
- Frontend: messages page, notification panel, unread count badge
- Server integration with module registration and routing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/erp-message/Cargo.toml

@@ -5,12 +5,16 @@ edition.workspace = true
 
 [dependencies]
 erp-core.workspace = true
-tokio.workspace = true
-serde.workspace = true
-serde_json.workspace = true
-uuid.workspace = true
-chrono.workspace = true
-axum.workspace = true
-sea-orm.workspace = true
-tracing.workspace = true
+tokio = { workspace = true, features = ["full"] }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+uuid = { workspace = true, features = ["v7", "serde"] }
+chrono = { workspace = true, features = ["serde"] }
+axum = { workspace = true }
+sea-orm = { workspace = true, features = ["sqlx-postgres", "runtime-tokio-rustls", "with-uuid", "with-chrono", "with-json"] }
+tracing = { workspace = true }
 anyhow.workspace = true
+thiserror.workspace = true
+utoipa = { workspace = true, features = ["uuid", "chrono"] }
+async-trait.workspace = true
+validator.workspace = true