Add private event invitations

db/migrations/11_event_privacy.sql

@@ -0,0 +1,15 @@
+ALTER TABLE concerts
+ADD COLUMN IF NOT EXISTS visibility VARCHAR(20) NOT NULL DEFAULT 'public'
+CHECK (visibility IN ('public', 'friends', 'private'));
+
+CREATE TABLE IF NOT EXISTS event_invitations (
+    concert_id INTEGER NOT NULL REFERENCES concerts(id) ON DELETE CASCADE,
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    invited_by INTEGER REFERENCES users(id) ON DELETE SET NULL,
+    viewed_at TIMESTAMP,
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (concert_id, user_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_event_invitations_user
+ON event_invitations (user_id, viewed_at);