Add config migration system

When config.json schema changes between versions, register a migration
function in src/config/migrations.js. On startup, loadSiteConfig() detects
the stored version, runs all migrations forward, and writes the result back.
Users never see the migration — it runs silently and the rest of the app
only ever sees the current schema.

Includes:
- v0 → v1: normalize dns from string to object
- v1 → v2: add dns.provider field (default 'technitium')
- Forward compat: configs from future versions left untouched
- Idempotent: re-running on already-migrated config is a no-op
- Safe: no user data is removed during migration

21 unit tests covering edge cases: null input, forward compat, corrupt
JSON, missing parent dirs, idempotency, full migration chain.

dashcaddy-api/src/config/site.js

@@ -1,10 +1,15 @@
 /**
  * Site configuration loader
  * Loads and manages site-wide settings from config.json
+ *
+ * Includes automatic migration from older config versions (see migrations.js).
+ * Users never see the migration — it runs silently on startup, writes the
+ * updated config back, and the rest of the app only ever sees the current
+ * schema.
  */
-const fs = require('fs');
 const { validateConfig } = require('../../config-schema');
 const { CADDY } = require('../../constants');
+const { loadAndMigrate, CURRENT_VERSION } = require('./migrations');
 
 const siteConfig = {
   tld: '.home',
@@ -21,9 +26,11 @@ const siteConfig = {
 
 function loadSiteConfig(CONFIG_FILE, log) {
   try {
-    if (fs.existsSync(CONFIG_FILE)) {
-      const raw = JSON.parse(fs.readFileSync(CONFIG_FILE, 'utf8'));
+    // Run migrations first — this handles config.json files from older
+    // versions of DashCaddy and writes the migrated version back to disk.
+    const raw = loadAndMigrate(CONFIG_FILE, log);
 
+    if (raw && Object.keys(raw).length > 0) {
       // Validate config and log any issues
       const { valid, errors: configErrors, warnings: configWarnings } = validateConfig(raw);
       if (log && log.warn) {
@@ -76,4 +83,5 @@ module.exports = {
   loadSiteConfig,
   buildDomain,
   buildServiceUrl,
+  CURRENT_VERSION
 };