Add user_sessions table to database schema

src/scripts/database/0003.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS user_sessions (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id TEXT NOT NULL,
+    bot_id TEXT NOT NULL,
+    answer_mode TEXT NOT NULL DEFAULT 'direct',
+    context JSONB NOT NULL DEFAULT '{}',
+    current_tool TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    UNIQUE(user_id, bot_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_user_sessions_user_bot ON user_sessions(user_id, bot_id);