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/config.php

@@ -4,19 +4,19 @@
  */
 
 function config_get(string $key, $default = null) {
-    $row = db_one("SELECT value FROM config WHERE `key` = ?", [$key]);
+    $row = db_one("SELECT value FROM config WHERE key = ?", [$key]);
     return $row ? $row['value'] : $default;
 }
 
 function config_set(string $key, string $value): void {
     db_query(
-        "INSERT INTO config (`key`, `value`) VALUES (?, ?) ON DUPLICATE KEY UPDATE `value` = VALUES(`value`)",
+        "INSERT INTO config (key, value) VALUES (?, ?) ON CONFLICT(key) DO UPDATE SET value = excluded.value",
         [$key, $value]
     );
 }
 
 function config_all(): array {
-    $rows = db_all("SELECT `key`, `value` FROM config");
+    $rows = db_all("SELECT key, value FROM config");
     $cfg = [];
     foreach ($rows as $row) {
         $cfg[$row['key']] = $row['value'];