feat(panel): add domain-admin role with per-domain authorization

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/store/migrations/0005_panel_users.sql

@@ -0,0 +1,26 @@
+-- Panel users and domain-admin assignments. Migrates the single admin row into
+-- a global user; drops the admin table.
+
+CREATE TABLE users (
+    id                  INTEGER PRIMARY KEY AUTOINCREMENT,
+    username            TEXT NOT NULL UNIQUE,
+    password_hash       TEXT NOT NULL,
+    role                TEXT NOT NULL CHECK (role IN ('global', 'domain_admin')),
+    dmarc_report_email  TEXT NOT NULL DEFAULT '',
+    created_at          TEXT NOT NULL
+);
+
+CREATE TABLE user_domains (
+    user_id   INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    domain_id INTEGER NOT NULL REFERENCES domains(id) ON DELETE CASCADE,
+    PRIMARY KEY (user_id, domain_id)
+);
+
+INSERT INTO users (username, password_hash, role, dmarc_report_email, created_at)
+SELECT username, password_hash, 'global', dmarc_report_email, created_at
+FROM admin WHERE id = 1;
+
+INSERT OR REPLACE INTO settings (key, value)
+SELECT 'dmarc_report_email', dmarc_report_email FROM admin WHERE id = 1;
+
+DROP TABLE admin;