Add complete database schema and ORM models (#4)

internal/models/model.go

@@ -1,14 +1,29 @@
 // Package models defines the data models used by the chat application.
+// All model structs embed Base, which provides database access for
+// relation-fetching methods directly on model instances.
 package models
 
-import "database/sql"
+import (
+	"context"
+	"database/sql"
+)
 
-// DB is the interface that models use to query relations.
+// DB is the interface that models use to query the database.
 // This avoids a circular import with the db package.
 type DB interface {
 	GetDB() *sql.DB
 }
 
+// UserLookup provides user lookup by ID without circular imports.
+type UserLookup interface {
+	GetUserByID(ctx context.Context, id string) (*User, error)
+}
+
+// ChannelLookup provides channel lookup by ID without circular imports.
+type ChannelLookup interface {
+	GetChannelByID(ctx context.Context, id string) (*Channel, error)
+}
+
 // Base is embedded in all model structs to provide database access.
 type Base struct {
 	db DB
@@ -18,3 +33,26 @@ type Base struct {
 func (b *Base) SetDB(d DB) {
 	b.db = d
 }
+
+// GetDB returns the database interface for use in model methods.
+func (b *Base) GetDB() *sql.DB {
+	return b.db.GetDB()
+}
+
+// GetUserLookup returns the DB as a UserLookup if it implements the interface.
+func (b *Base) GetUserLookup() UserLookup {
+	if ul, ok := b.db.(UserLookup); ok {
+		return ul
+	}
+
+	return nil
+}
+
+// GetChannelLookup returns the DB as a ChannelLookup if it implements the interface.
+func (b *Base) GetChannelLookup() ChannelLookup {
+	if cl, ok := b.db.(ChannelLookup); ok {
+		return cl
+	}
+
+	return nil
+}