feat: implement chat and conversation history management with database schema and API endpoints

db/schema.sql

@@ -208,3 +208,30 @@ CREATE TABLE IF NOT EXISTS system_statistics (
 
     created_at TIMESTAMPTZ DEFAULT now()
 );
+
+CREATE TABLE IF NOT EXISTS conversations (
+    id UUID PRIMARY KEY DEFAULT uuidv7(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    mod_id UUID REFERENCES users(id) ON DELETE SET NULL,
+    status SMALLINT NOT NULL DEFAULT 1,
+    closed_at TIMESTAMPTZ,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS messages (
+    id UUID PRIMARY KEY DEFAULT uuidv7(),
+    conversation_id UUID NOT NULL REFERENCES conversations(id) ON DELETE CASCADE,
+    sender_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    content TEXT NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS chatbot_histories (
+    id UUID PRIMARY KEY DEFAULT uuidv7(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    question TEXT NOT NULL,
+    answer TEXT NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+