Add models package with embedded DB interface pattern

- internal/models/model.go: DB interface + Base struct for all models
- internal/models/channel.go: Channel model with DB access for relation queries
- Database.NewChannel() factory injects db reference into model instances
- Uses interface to avoid circular imports (models -> db)

internal/models/model.go

@@ -0,0 +1,18 @@
+package models
+
+import "database/sql"
+
+// DB is the interface that models use to query relations.
+// This avoids a circular import with the db package.
+type DB interface {
+	GetDB() *sql.DB
+}
+
+// Base is embedded in all model structs to provide database access.
+type Base struct {
+	db DB
+}
+
+func (b *Base) SetDB(d DB) {
+	b.db = d
+}