feat: switch from MySQL to SQLite for zero-dependency deployment

- Rewrite db.php to use SQLite via PDO (no external DB server needed)
- Rewrite schema.sql for SQLite (AUTOINCREMENT, CHECK constraints,
  triggers for updated_at, ON CONFLICT instead of ON DUPLICATE KEY)
- Fix config.php upsert to use ON CONFLICT syntax
- Fix index.php: tag filter via json_each, vote upsert via ON CONFLICT,
  and fix protocol create route (was returning 405 for POST /api/protocols)
- Fix yaml.php seed import: replace invalid false callbacks with fn() => null,
  fix yaml_parse pos argument (-1 -> 0)
- Update Dockerfile: drop pdo_mysql/mysqli (pdo_sqlite is built in)
- Update CloudronManifest.json: remove mysql addon, bump to v0.3.0
- Update README for SQLite deployment

api/yaml.php

@@ -83,10 +83,12 @@ function yaml_array(array $arr): string {
  */
 function parse_protocol_yaml(string $yaml_text): ?array {
     if (function_exists('yaml_parse')) {
-        // Harden against PHP object injection via !php/object tags
-        $data = yaml_parse($yaml_text, -1, $ndocs, [
-            '!php/object' => false,
-            '!php/const' => false,
+        // Harden against PHP object injection via !php/object and !php/const tags.
+        // The callbacks must be valid callables; null them out to neutralize deserialization.
+        $neutralize = fn() => null;
+        $data = yaml_parse($yaml_text, 0, $ndocs, [
+            '!php/object' => $neutralize,
+            '!php/const' => $neutralize,
         ]);
         if (!is_array($data)) return null;
     } else {