feat: added invite table

internal/database/migrations/000003_create_invites.up.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS invites (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    inviter_user_id UUID REFERENCES users(id) ON DELETE SET NULL,
+    invitee_user_id UUID REFERENCES users(id) ON DELETE SET NULL,
+    code VARCHAR(128) NOT NULL,
+    consumed_at TIMESTAMPTZ,
+    expires_at TIMESTAMPTZ NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE UNIQUE INDEX idx_invites_code ON invites (code);
+CREATE INDEX idx_invites_inviter_user_id ON invites (inviter_user_id);